Personally I think that critics have exaggerated in their reviews of this movie. Sure, it's a fun watch, entertaining and emotional - it's very different and interesting. But that's about it. There's nothing oh-God-so-fantastic about 3 Idiots. Critics have a tendency to praise Aamir's movies beyond fabulous words in their vocabulary - and yes, he does often deserve the praise as his movies are always very different, very interesting, and the characters he portrays are usually intriguing. 3 Idiots showed no new or extraordinary side to him, though, despite his character being awesome.
3 Idiots has some interesting themes such as the struggle most people face when trying to achieve their dreams. Their fight for survival in an environment where there's no room for the weak - it's basically as Darwin would put it; survival of the fittest. And in 3 Idiots, it's survival of the smartest and most intelligent of them all.