Iran stands on the cusp of a revolution. It may seem presumptuous to state so; after all, can a few thousands or millions on the street really shake a 30 year old regime? I say 'shake' because the stated aims of the protesters is not to uproot the 'Islamic' part of the Islamic Republic of Iran; it is to emphasize the 'Republic' part. And that would be an important step forward.
Ever since the Revolution of 1979, the conservative factions of the Iranian people have systematically silenced the more reformist and liberal voices in their society. A nation that was once at the vanguard of globalization (the Persian Empire, the Safavids) now turned its back on the world and chose to stay secluded within the walls. They have denied their people the right to choose their leaders, those who will rule the nation in their name; true, they have elections, but the candidates aren't always those who want to serve; they are those who want to serve and are 'permitted' to serve. Permitted by whom, pray? For in a democracy, in a Republic, that was founded on the rage of the people against a tyrannical king, who save the people can permit someone to serve them? To presume otherwise is to subvert the will of the people.
True, Iran isn't just another Republic; it is a theocratic republic, guided by its clergy, the 'incorruptibles'. Whether they truly aren't susceptible to human emotions and prejudices is a matter of debate, but even they cannot be entirely trusted not to take care of their own power source at the expense of those whom they are to 'serve'. It's a classic case of a conflict of interest: to loosen up would endear them to the masses, but may mean a reduction in their authority; to tighten down may earn them the undying hatred of the people, but will ensure that they enjoy absolute power for their lifetimes (or till they can muster the brute-force to silence their critics).
Critics of Mousavi's protests state that he, being just as much a creature of the Revolution as Ahmadinejad, cannot be trusted with genuine reform. Mousavi's intentions are best known to him, but the courage and the determination that he has shown to challenge the establishment, to suggest that there's something rotting in the system, is admirable and worthy of being emulated. Gorbachev too was a dyed-in-the-wool Communist, but his perestroika and glasnost were genuine attempts to reform the Soviet state before it imploded on its own people; that they failed were both his and his people's misfortune. Mousavi promises more, because the people back him, and yet the perils of failure are even more.
What is more important is that for the first time the reformists have gained a slightly upper hand over the conservatives. Khatami received a mandate but couldn't get past the clergy's machinations; his failures meant that Ahmadinejad seemed a better bet the last time around. This time, it's time for change, and the Iranian people must raise their voices, for their lives, for Iran.
P.S. I find this song to be particularly poignant and pertinent to the situation. Especially the lives "We're not going to live in silence; we're not going to live in fear. This time we know we all can stand together. We have the power to be powerful, believing we can make it better."