I tend to agree; part of my interest in metal stems from the fact that bands seem to be more willing to branch out and experiment due to the music's 'underground' status. (Though there are some people in black metal subculture who deplore anything that doesn't sound like Darkthrone's Transilvanian Hunger, but smurf them). I definitely agree with finding the lyrics intriguing when I can understand them; that said, a lot of bands I listen to don't even sing in English (Moonsorrow, Arkona, early Enslaved, and so on).

as long as we're talking about metal bands who use mythology and history as bases for lyrics I'll have to mention Nile, who include essay-length pieces about the historical basis behind their songs in their liner notes, and Satyricon, who aren't technically Viking metal but incorporate a lot of historical basis into their lyrics, especially in the early stuff (note that the sound quality isn't that great though). And then there's Pan.Thy.Monium, who just invent their own mythology entirely.