is there a specifc law that requires a candidate to produce X # of years?
there isn't. I doubt there is anything illegal about his returns. I expect he just doesnt want Americans to know that, like most of the wealthy, he utilizes legal methods to pay very little in taxes.I don't know what the big deal is about that, but I have a hard time believing, like you, that there is anything illegal hidden on his returns.
Romney's father set the precedent of releasing several years of tax returns, and in general, most candidates have. Romney himself demanded Kennedy release tax returns in a previous race. He also required his VP candidates to release to him multiple years of tax returns. However, what Romney thinks other people should do apparently doesn't apply to him. That I find kind of odd.
At this point, his releasing his tax returns won't change my mind about voting for him but it would comfort me if he became president. I also think independents might actually care, because it is indicative of the kind of transparency Romney might provide the American public.
The narrative would change to, what is he still hiding since it took him so long to release them. Nevermind what's important.
For some, but not for most. I think if he released them the VAST majority of people would just say, "oh, okay, he's just so filthy rich he thought we might find that offensive or something, whatever." It'd go away completely - assuming he didn't just release 3 lines of them or something.