Listening to metal for the vocals (or lyrics, for that matter) is like watching an action movie for the story. Yeah, they're there, but they're not really the point, and if you can't get over it, well, you're missing out on all the other good stuff. Actually, I'd say heavy metal pretty much is the musical equivalent to action movies, period. Yeah, sometimes it's dumb, sometimes there's not much variety (well, within the subgenres - most death metal sounds the same, IMO, for example, though within "metal" itself there's a huge variety it'd be dumb to dismiss outright), but who cares? Sometimes you just have to say "fuck you" to subtlety and just enjoy the sheer thrill of the balls to the wall action and craziness going on.
And metalcore in general isn't that good. Especially the more commercial stuff, which utterly fails at mixing "hard" and "soft" well. If there's one thing I hate in music, it's jarring, badly done transitions between "hard" and "soft" sections.
As for alternative, for the most part, I like a lot of it, but it just lacks the sheer, visceral thrill metal has for me. There's just not enough energy in it, and so much of it just seems too self-conscious, like "hey look at us we're being totally natural and artistic and stuff." Attitude (not like "attitude" that means badass or something like that, but the other kind, the more general kind) is important in music to me and the attitude of a lot of alternative rock turns me off.
If my post seems incoherent or something, well, I'm dead fucking tired and have been for more than a week now.