1) The stuff that happens in your teens is special because of your age. That may well explain why I think The Beatles (and the music of that era) was so special.
2) The music from the 60s really was special. There was certainly rock and roll well before The Beatles, and it grew out of black music, and blues, and can be viewed as nothing really new. And yet: it really was a significant break from the past in the global adulation for their music and the quantum change in tastes that they ushered in.
Not to minimize Nirvana (whose music I love) or metal, or anything else, but the musical examples that you cite were simply not as globally shattering as The Beatles. They just weren't. They were variations within the world of rock and roll. The only comparable change I can think of is the rise of rap music. Which I cannot stand, but I recognize how it changed the world in the same way that The Beatles did.
