Had things been a little different, I firmly believe that Nirvana could have been yet another divisive artist in pop music history. The 90s were, thanks mainly to the bizarrely large success of "Smells Like Teens Spirit" and Nevermind were the home of the now collapsed alternative rock genre, along with his sibling/child sub-genres of grunge, No Wave, post-grunge, noise rock (to a minimal extent), Brit rock, and post-punk (all those Joy Division rip offs basically; Nick Cave has never been mainstream). This gave way to rap and, later still, hip-hop, club, and contemporary R&B, essentially a backtracking to Michael Jackson. Somehow, the alternative genre experienced a disco-esque lifespan; a monumentally large success matched in degree only by its brevity.
A large part of this was no doubt the apparent instability of the genres major groups. Nirvana dissolved when Kurt Cobain passed away, Alice In Chains went into limbo when Layne Staley overdosed, and none of the other major groups in the grunge wave seemed to have any staying power (Stone Temple Pilots and Pearl Jam only really made two good albums each, Soundgarden fell through when Chris Cornell went crazy, Temple Of The Dog had similar problems with deaths, etc).
The other issue was one of size. Even at its peak, grunge was a very tiny genre compared to other mainstream flavor-of-the-months. Its fellows in the alternative rock umbrella also suffered from the simple fact that they identified themselves as alternative. Melodic hooks served as an exception to oftentimes off-putting cacophonies of sound, especially in the noise rock and No Wave movements captured by groups like Sonic Youth and My Bloody Valentine. Each of these groups had a few radio-friendly unit shifters peppered in amongst a sea of feedback and distortion, like "Where Is My Mind?" on Surfer Rosa. They didn't want staying power; they wanted obscurity, like punk before them.
Nirvana seemed the major exception to this rule. While Layne Staley seemed to have a similar love for sheer melody, Kurt Cobain embraced it in a way that no other alternative musician ever did. Pearl Jam was the closest band in terms of approach but, lets face it, they never had anything on Nirvana. The best you can say about Eddie Vedder and his merry men is that they had better solos.
I'm sure music fans are getting in a tizzy reading this, but you have to look at the facts. Even on Bleach, probably the least radio-friendly album Nirvana made, you can see Cobain's touches of melody. The only real differences between that album and the landmark Nevermind are production and approach; the group's debut was tailor made to fit into what was then considered grunge at the behest of their label, Sub-Pop, while Nevermind was more unsupervised due to the recent success of Sonic Youth. On that album, and even on the deliberately uneven In Utero, melody was king. It was that contrast, along with the terraced dynamics that Cobain ripped from the Pixies, that made their music interesting.