When they began, they were a call for justice, from the people to those who really hold power, such as Grand Ayatollah Khameini. The protests were a petition for his supreme judgement to mandate another election. They were asking for a legal, legitimate action, one which would occur from inside the Iranian political structure.
He failed his people.
Instead, he supported the conservative Ahmadinajad (big surprise there, being that if the election really was rigged, Khameini had to have signed off on it). He could have listened to the people in the streets. He could have heard the voice of the citizens of Iran. Instead, as often happens when someone is in power, he sought to further his own agenda.
My guess is that Iranians were tired of the rhetoric and the struggles, tired of being cut off from the world, maligned by the words of their version of George W. Bush. The protests have changed dramatically over these 10 days or so. They started peaceably, with hundreds of thousands of citizens walking in silence that astounded the world with their poise and solemnity. People quoted Ghandi and Martin Luther King, Jr., and tied green ribbons around their wrists. Then they were declared illegal, and the government responded to them with violence. When Khameini ignored the protests, the people realized their fight had to go straight up to the top. When the police started to arrest and kill, the call changed. Mousavi is no longer in charge of the "Sea of Green" (He's been incredibly silent for the past day in fact. I think he realizes what is happening). The rhetoric of the protestors has changed too. Gone are the calls for peaceful resistance. Instead, Twitter is populated with recipes for home-made napalm and increasingly violent videos are surfacing on YouTube.
For almost 10 days, the citizens of Iran have taken to the streets. If this was just about an election, they would have gone home by now. The reform that these protesters seek doesn't stop at Mousavi vs. Ahmadinejad. If it did, they wouldn't be calling for the death of their supreme governmental leader, or posting close-up pictures of Basij and asking everyone to identify them and their home addresses, saying "we know you and we will kill you".
As the temperature rises in Iran, something has become clear: This has become bigger than the elections. It is about absolute disenfranchisement and theocracy. It can only end with blood, that of the Mullahs or that of the citizens that fight against their tyranny. I, for one, support the citizens.