This is such a huge question that I can’t possibly begin to answer all of it unless I wrote a book. Since I sadly don’t have time to write a book, I’ll answer with a few things off the top of my head. (For the record, my con experience has been anime cons in Northern California [barring trips down to AX in 2003 and 2016], starting in 2003, but seeing parts of anime fandom and cosplay and con culture through the internet a year or so before that. I was in my early teens when I started attending cons, so my experience may differ a bit from someone who was older when they started.)

Programming:
- Focus on anime releases. This was long before streaming websites came on the scene (like Crunchyroll, Netflix steaming, Hulu, etc.), and just after/on the tail end of the transition to DVD (I had just missed the generation of sketchy VHS tapes copied off of a friend, with really bad hard subs or dub-only, though I do remember encountering this type of thing). DVDs were expensive and didn’t come in multi-packs (now I can go down to a local record store and get the full series of something on just a few DVDs for the cost of a single DVD with four episodes or so back in the day), so you had to buy each one individually. Usually the first DVD in a series had a box so that you could fill it up as you bought more, and a complete box set sold in stores for quite a lot (well over $100). There was anime on TV, and this was really the heyday of that – Toonami, the height of Adult Swim anime (it was around this time that thy made a specific anime programming block), the Sci-Fi channel, the International Channel, etc., but the way to see things not on TV? Buy a DVD (or borrow from a friend), or watch in a con viewing room. This let people see the newest releases and test out series before dropping a ton of cash on them. (Side note: I found that I also have an ADV catalog dating 2003, which is amazing. I might still have the promo DVD that came with it!)

- Fandom-specific panels were extremely rare. In-character Q&A role-playing panels were unheard of until…Hetalia, maybe? And then Homestuck after that really made those popular, but those didn’t gain popularity until around 5 or 6 years ago. Before that, if there was a fandom-specific panel, it was usually one of two things: someone who worked on the show giving a panel about it (which was information, Q&A, and their personal experiences of working on the show…these panels are still around), or a fan-run panel that was mostly informative, but may have also included games, giveaways (and the prizes were typically pretty sweet), and limited joking around in-character, but the focus was usually on OOC information providing. More “history of…” type panels than you see today.

- Lots of how to panels. Many of which were focused on how to create your own anime or manga, and ran the gamut from really basic intro panels to in-depth panels on one specific topic. Also more cosplay how-to panels than you typically see now. Lots of fanfiction workshops. A good amount of comedy panels, though these were usually general to anime fandom and not specific to a single fandom. More meetup-type panels than you see today – while organized cosplay gatherings have been going on for as long as I can remember, there used to be actual panel rooms with time dedicated to things like a meetup for the people who post on the con’s forums, or for other non-cosplay things. This type of thing would be organized elsewhere now.

Merchandise:

- Mostly anime merch. Posters, wallscrolls, pencilboards (can you even still get these at cons?), clear file folders, keychains, pencils, etc., Very few booths with Western merch, though usually at least a few with live-action Asian films, Asian music (I haven’t seen a booth selling mostly CDs in a very long time!), etc. Lots more DVD and manga booths. (Keep in mind that Tokyopop became Tokyopop only in 2002, so this was really the start of having manga sections in bookstores and such.) Doujin booths. Tons of booths had $1 discount manga bins, where you could often find some gems, but those went by the wayside probably circa 2008.

- Booths sold magazine cutouts. Photos of Japanese celebrities (usually Jrock/Jpop/visual kei stars). That type of thing. They may have had signed products as well. There would usually be an entire booth or two dedicated to this type of product. Mostly for people who were interested in a certain manga artist or musical artist or anime but didn’t want to pay to import an entire magazine for one photo or one article. I’m sure the prices were jacked to the moon, but I never really browsed these booths. I don’t think I’ve seen this in nearly 10 years.

- All the same kawaii booths and snacks and such, though it seems like it was often more separated out (rather than every booth having a few Sanrio plushies, there were three or four booths with a ton of them), and it feels like a lot of kawaii booths sold the most snacks, but there’s one booth in particular at Fanimecon that vanished around 2009~2010 or so that was huge and mostly kawaii things (plushes, pencil cases, etc. with Sanrio and San-X characters) and a huge selection of snacks that I could be thinking of. Snacks were, as always, overpriced. That much hasn’t changed. (So glad I now have a car and a 99 Ranch near me ;P)

- General anime fandom merch was popular, and reached peak popularity in the mid to late 2000s. A few booths had cosplay goods, but pre-made cosplay wasn’t as big as it is now with Taobao and everything. These booths usually also had cheap lolita fashion knockoffs and such, and actual brand came into the scene around the late 2000s. You could usually find at least a couple of booths with general weeby clothing, like “Chinese dresses” and cat ears and cheap corsets. Lots of places had those fleece animal hats. Lots of animal tails, fishnet, chains, leashes, rave kandi, etc. worn in the halls, and sometimes sold. Yaoi paddles. All the things usually associated with early to mid 2000s anime con culture. You can still find plenty of this category of merch, though it seems like there’s more of it now, it’s proliferated into general nerddom and not anime-specific, and the exact type of item has changed with the tastes of the fandom (so now it’s snapbacks like the one in the my icon or my Sensei one I wear sometimes and not felt hats shaped like Kyo from Fruits Basket, but the idea is the same).

- AA seemed to have less focus on prints. More buttons. More crafts. More handmade clothing. Prints were still a thing, but it wasn’t like walking into the AA today and being met with a wall of prints of all the popular (often Western) series.

Guests:

- More Japanese guests. This was the heyday of anime becoming popular in the West, and Japanese companies seemed to be more willing to send over Japanese seiyuu and such. Fanimecon in particular was well-known for its music scene, and for getting numerous Japanese bands to come perform (though these days everyone complains about the lack of guests at this con generally, but it still seems like a general trend that it’s harder for cons to get Japanese guests). American dub VAs have always been around at cons, as well. 

- This may be a misperception because I never really paid that much attention, but I feel like creators, character designers, and artists were more popular as guests than they are now. A character designer for a popular show was just as much a draw as the seiyuu who voiced the protagonist. It seems like VAs are now the main draw. But again, I never paid that much attention, and still really don’t, honestly. (Of course, a big-name artist or creator would still be a huge draw, like if a con managed to land CLAMP or Yoshitaka Amano or someone, and a few years back, JX brought out some character designers that were hugely popular, but that elt like an exception rather than the rule.)

- Cosplay guests were unheard of.

Cosplay:

- Al the usual old cosplay things. Weird fabric choices (you can tell who was trained in cosplay during these years because we often still have a knee-jerk reaction to the words “satin” and “crushed velvet”), like super super shiny satin that’s limp and can’t hold a leat being use for a sailor fuku kind of weird. No wigs. If you had a wig, it was amazing. Wigs were either extremely expensive but good quality or far overpriced for horrible quality. The only unnatural colors were neon. Everything was frizzy. Wig dyeing became a thing around this time, but white wigs were hard to find, and most people still couldn’t afford the time or money to bust open tons of Sharpies and soak them in rubbing alcohol and attempt to dye an overpriced wig with the resulting mixture. Wigs became much more common towards the end of the 2000s (2007 or 2008 or so, I’d guess), once online shops started making and carrying more cosplay-specific wigs, and once it was easier to get them due to Paypal and increased web security. Colored hairspray was common, as was dyeing or cutting hair to match a character. Armor was cardboard or non-existent, and once people started using craft foam that improved a lot, but armor has always been impressive as hell.  Cosplay in general has really increased in quality since then, and store-bought cosplays are now common and acceptable, where before, you’d often have to go through a commissioner to get one. There’s more focus on accuracy now, where before, people would often modify thrifted things until it was close enough.

- That said, while there was a lot of what would now be considered ‘bad’ cosplay, there was a lot of really good cosplay, too. There was a bigger focus on fabric craft than on things like armor and props, though these existed. There were some god-tier cosplayers who had giant ballgowns, wings, styled wigs, props, etc. etc., and that was always amazing. There was a lot more experimentation, trying to figure things out (since there weren’t as many resources as there are now), and making do with what you had, and less focus on being the ~~perfect~~ version of a character, or being ~~someone’s headcanon version~~ or whatever. Hand beading and embroidery was usually pretty amazing. Well-made costumes with clean construction were usually god-tier. 

- Types of cosplays were different. Usually anime or Japanese in origin (Japanese video games have always been popular at anime cons), and very few Western-series cosplays. There was more outright hostility towards people cosplaying from something American, though I never really understood that. If there was a Western-series cosplay, it was often done as a joke – like the group of 300 cosplays running around with painted-on abs in speedos at Fanimecon right after that movie came out. Fewer meme cosplays, though these have been around for as long as there have been memes. More offbeat humor cosplays not based on memes, like dressing as a Pocky box, box Gundam, or Tetris pieces. More people holding signs, like “free hugs” or “will glomp for Pocky” or “will yaoi for money,” etc. Lots of people in fishnet and Tripp pants walking each other on leashes. Visual Kei cosplay was ever-present and ever-impressive. Less J-fashion because it was less available, but usually there were still people in lolita and such around. A couple of years into my con-going life was when wearing Naruto headbands bought in the dealer’s hall and cat ears and whatever other fandom merch you could pick up (usually in the dealer’s hall or at Hot Topic) became really common.

- Sexy cosplay has always been a thing, but beauty standards were different. This was before circle lenses, before most people figured out false lashes and contouring, etc., so it was more natural-looking, and it was things like Lum cosplays or Cutey Honey and not things like Worbla boobplate bikini armor. Not better or worse, just different. Less focus overall on looking just like the character, but of course, young and attractive women who looked like their character (or simply looked good in the costume) would get a lot of attention, and there was always a perception of “cute young Asian girls get all the attention,” whether it was true or not. I remember a bit of a scandal at Fanime…2006, I want to say? where a cosplayer went as Chocolate Misu with only a bit of double-sided tape under the suspenders (no bodysuit or anything). I’m not sure if that would still cause a scandal or not, unless she had a slip. 

- There’s always been drama, but it used to stay localized or only really talked about on forums. Now with social media, there’s more widespread drama. People have been just as supportive and amazing with sharing techniques and giving help and such as they’ve always been, though back when the community was smaller, it was more tight-knit.

- Photoshoots were less of a thing. People did them. But most photos taken were hall shots. This was back when digital cameras were first becoming available to the average consumer, so there were a lot of disposable film cameras still.

General things:

- This might be the taint of nostalgia, but people seemed to be /really/ into it. Probably because in those days, anime cons were something you really had to seek out, as compared to today when the con market is oversaturated and everyone and their grandma knows what ComicCon is. There was also less anime on the market, and while you had your popular series, it was likely that everyone had at least heard of most series that you would be talking about, if they hadn’t seen it themselves. Anime was also often from a mix of eras, since some series only became popular in the US after being brought over, and during the early 2000s boom, a lot of older series (Dragon Half comes to mind) were first being officially licensed and widely shown, even if they were made a decade before. It seemed like you could have a good chat with anyone at the con about your favorite series, or at least a series you had a common. Now there’s a proliferation of series, which is fantastic, but aside from a few really popular FotM series, people may not know of a lesser-known series that you love, or may not know an older series.

- As said before, less Western fandom and general geekery, and more anime-specific fandom. People often were into other things as well, but you would rarely find someone who went to an anime con and barely watched anime and was more into Western cartoons (there was also a weird sense of anime elitism that probably continues to this day, where people thought that anime was superior to Western cartoons) or comics or video games, whereas now, the lines between genres at cons seems to have blurred some.

- I was part of sort of the first generation of teenagers to enter the scene, so there were still a lot of older people at cons, but also a lot of teens and early 20s. It seems like the average age of anime cons had trended downward since then, though older folks are still around.

- A lot of general weebery going on. Lots of cons have now banned signs, yaoi paddles, spin the bottle, dance circles, etc., and there’s still TONS of young kids and teens with all that young energy, but my generation is the one that got a lot of that banned for the later generations. Sorry. ;P Glomping was a HUGE thing then, and thankfully had died down because unwanted glomps were always terrible.

- DDR was enough of a thing that one of the old programming guides I was just flipping through (I want to say AX 2003) actually had a DDR etiquette guide alongside the general one (which was mostly about hygiene lol). Also: para para dancing.

- Less “cosplay is not consent” type information out there. I’m sure harassment has always been a thing, though thankfully I never experienced it until I was older (and of legal age). There was less awareness of tht type of thing.

That’s about all I can think of, and I’ve just spent about two hours writing all this up, so I’ll stop for now. I’m sure there’s a lot more than can be said. And, of course, this is just one person’s perspective, and there may be regional differences or age differences etc. etc. etc. 

I hope that gives you some idea, Anon! If you have more specific questions, I can answer them tot he best of my ability. :]