I never really got into Tool, though I do like a handful of their songs. I find their albums to be a bit too lifeless. As for Nightwish, that's not really my style of metal at all, and I don't blame anyone who can't get past Tarja's operatic vocals.
It's funny, I sometimes put on U2 stuff I haven't played in a while, and in my mind I've got this memory of how I thought it was such a heavy, rockin' song. And ... it's just not. That's not to say I think any less of the songs, as I usually don't. I was quite let down when I heard Miracle Drug lately, though. I remember the solo once felt like a fantastic release, the part where Edge really lets loose was almost transcendent ... but I felt nothing and it felt like he was playing so weakly.
I think a lot of people don't realise how diverse metal is. I think most people know of thrash and power metal, like Metallica and Blind Guardian. I am very much into the atmospheric and avantgarde stuff, much of which draws on unusual musical theories or folk music. In many cases, the folk influence is quite unrecognisable, especially to an outside listener. The general sound and asthetic is really an acquired taste, but it's certainly not all bands with stupid names like Fuck I'm Dead (real Melbourne band). There's some quite intelligent stuff. Orphaned Land, for example, are an Israeli band who bring together the three main religious traditions of their country to find common ground and promote inter-religious harmony.
OK, I'll stop before I really get going! Probably time to sleep anyway ...