Well I was driving along, listening to the new All That Remains album, This Darkened Heart some time after I had read this post, and listening to the first track, a little before the solo section I found the simple answer to the question. Its just a damn awesome combination and there's no way around it.
Aside from that I just want to say that its music that you can tell people put time into that attracts me, for the most part. Classical and metal just happen to fit into that category. Of course there are metal bands that throw crap together really quickly, but you can hear it in their songs, and I don't like them.
Jazz is a whole other category, where the musicians have mastered their instument and countless scales and modes and chord relationships. I would love to get a couple jazz musicians together and play some metal, in a similar style of four-bar blues. That would fucking rock, imo.