I also can't stand a lot of nu-metal, etc...but my main thing is I just can't stand music with no heart put into it. Sure, some people may dislike the early black metal of the 90s and late 80s, but it was played with conviction(I love it.) It's not as much genre for me(though I do listen to mainly either underground extreme metal of all kinds, or the 80s stuff I grew up with), but whether it has heart. And about 99.9 of these nu-metal acts and such just have no heart or conviction in their music. Not even talking about lyrics; some bands sound like they just don't care and are just out to make a buck. Sure it's a living for some bands but you can just listen to a band and tell just how much they are into what they are doing. For instance, when I listen to Dissection, I can just feel how into it they are; which is why they are one of my favorite bands. When I listen to CoB, to me they sound like they are really love and are having fun doing what they are doing; not to mention the music kicks ass on top of it. And all of the bands/cds that I like, I just "feel" something in them that makes me think "yeah, they do really care about their music." The Ramones, as simple as they were musically, you could hear they really liked doing it, also.
Now, I really can't stand rap and R&B, just on a musicial level. And just to toss out some band names, Limp Bizkit, Linkin Park, etc, etc....can't stand stuff like that. Like I said; no heart whatsoever(not to mention no talent)