All I can think of for most metal is "That's an awfully good background. Too bad I have to listen to this throat tearing gibberish."
Easy solution: stick with the less abrasive forms of metal.
Anyway, to answer the question, I look for different things depending on the genre. For the most obvious examples, let's look at prog rock and heavy metal. For prog rock, I'm looking for complex musicianship that can still sound good. For metal, I love the power behind it. Or at least, that's the short version.
As a massive metalhead, I feel that each sub-genre has its own appeal. While they all share the power weight, how they use it is different.
Power metal: The smooth melodies clash wonderfully with the heavy guitars and often combine with orchestration and synths to create a sound that is "big." This is especially true of bands that use operatic vocalists like Kamelot or Blind Guardian. Especially Blind Guardian. I mean, what do you expect when a band will put over a hundred layers of vocals in one song?
Prog metal: Same kind of appeal as prog rock, only with the weight of metal.
Thrash metal: The anger. The snarled vocals and blunt guitars work for an angry sound that emphasizes metal's crushing weight.
Melodic death metal: Takes the outright fury of death metal, but adds in melodies to take the edge off that turns me away from most straight death metal.
It should be noted that these are only the forms of metal I particularly like. There are bands outside of these sub-genres I like. It should also be noted that these four sub-genres have something in common: a strong focus on musicianship. I like skilled musicians, what can I say?
