Weetbix: just read Wrathy and Spawny's descriptions of Nirvana there to see why I think their influence was negative. ie. "dischordant noise" and "droning voice". Most rock bands since then have had droning or yearning vocalists and sound like just a wall of noise noise rather than having tight riffing and melodic lead guitar work etc.. like rock music did before Nirvana hit it big.
I just think the things that their success made unpopular/uncool for bands to do or sound like were the things that made rock music so great (such as melody, fun and excitement!). Nirvana seemed to be targeted at troubled teens who needed to express their unbalanced emotions and feelings etc haha! Fuck dealing with those things, just escape from them with something fun!
I know you can't blame Nirvana themselves for it, they didn't go out to write an album that would deliberately have that effect on what rock music would turn into. I'm just saying the influence of that album becoming successful is what did it, hence it's influence being negative rather than positive.