In Iran, some people apparently tried to steal an election. This caused protests, and a huge wellspring of support. Then some people tried to throw some cold water over the exuberance, claiming that Mousavi was no Gorbachev, and that history would repeat itself as Mousavi would reveal himself to be a stooge for the Iranian higher-ups.
Does history repeat itself? No, it doesn't, and Gorbachev is the best example of that. He spent his entire career with the Party, and was a committed party man. There was virtually nothing in his background that would suggest that he would embrace a path of reform; yet he did. Deng Xiaoping opened China, Manmohan Singh embraced economic reforms in India, Nixon went to China.
People don't always do the things you'd expect them to, even based on their past. Nor do historical events keep repeating themselves--we just tend to remember the times events echo, and tune out the wild swings that go on. The Islamic Revolution there kind of came out of nowhere, and used revolutionary new communication technology (cassette tapes).
This isn't to say that Mousavi, if he ends up winning, will or will not become a reformer. We just don't know. And that's about all you can say.