I do agree that a lot less rock and metal does talk less about violence, death, and whatever else fits in the rap genre. They talk more about polotics, the world in general, relationships, fantasy...
But... Nightwish is one of those death metal bands... I mean, they probably don't fit into the "death metal" genre so much (some sort of metal though). But the lyrics in some of their songs are hell depressing and on one of the albums the band members even said that they were worried about the lead singer cause' of the depressing lyrics he was writing...
Never the less, I still like Nightwish and just because I listen to them doesn't mean I'm depressed.