I would argue, even though Nirvana was a very simplistic band (which tends to be one of the main arguments against their alleged greatness), that they are not overrated. The music is great, even though I myself prefer Soundgarden from the grunge erea (while the two share little in common music wise. It's like the NWOBHM phrase all over again), but the cultural impact Nirvana had is profound and justifies the esteem in which they are held. Music feeds off the fan reactions. Music can be very simplem but highly efficient. Kinda like Creedence Clearwater Revival, another equally simplistic band. Their songs do more for me in less than three minutes than other bands might do in 10 and while they are very simple they have me digging it far more than most progressive rock bands.