I can only speak about metal, not because it's the only genre I listen to (which is not even close to reality), but because most of my close friends listen primarily to it.
What I usually read about and experience (from other's point of view) is that metal listeners are usually:
- introspective
- close-minded
- stupid / lacking intelligence
- violent
Also, when it comes to music taste, they're sometimes close-minded too, meaning that they like metal and only metal, or similar genres like hard rock.
About stupidity, it couldn't differ more from reality. The smartest and more open-minded people I know are metal fans, and you can have a nice debate with one about almost any topic.
Being violent never came into my mind actually, even if some of them "look" violent, judging by the clothes and physical appearance, I never felt threatened by any metal fan, and they were always very kind to me and everyone.
I'll try to talk about other genres later, of course it won't be much accurate, but this is good for now, since it's the most liked music genre in this forum.