The problem with things being over-rated is that music/movies/games are often rated against their peers. So yeah, when someone looks at Nirvana NOW, they seem to sound like a lot of other bands, but if you look at Nirvana in 1989, they were one of only a handful of bands with such a sound and arguably the most talented of them all. They might not have been amazing or anything, but they were so different from the regular 80's sound that they jumped out at everyone. They weren't really the first grunge band or even the best, they just caught it at the right moment.
Nostalgia is what determines a lot of people's opinions about entertainment. You can't play a game that is 15 years old for the first time and appreciate it in the same way as someone who played it at release. I fucking loved Ultima Online and still think of it as one of the best, most innovative games ever...but I promise you someone that tries to play it for the first time today will think it is garbage (Well, Trammel did ruin it).
For me, the most overrated thing I can think of is Lord of the Rings (books). So. Fucking. Boring. They walk, they camp, they're scared, they walk...zzzzzzzz.
