I kind of expect different things from different genres of metal (I'm narrow minded and often only listen to metal, but am open to other music).
For nu-metal, It's really hard to sound good because most of it's the same boring crap. Some of it's more catchy, some of it's actually pretty good and the musicians have some potential at being better. It's a simple genre that doesn't rely at all on technicality. I don't expect much from it.
For metalcore, I expect fast music with a fair amount of technicality. I don't want a band to overcompensate for lack of creativity with a 6 minute breakdown, but I don't want them to negate the breakdown. Breakdowns, I think, can be just as effective as any interlude, if done in moderation.
For grindcore, I just want craziness. No musicianship, no thought. Just noise. LOUD, distorted, dissonant noise. That's what grindcore is to me.
For the rest of metal, I'm looking for a different sound in each band. In nu-metal or metalcore, you could have, like, a hundred "System of a Down" ripoRAB or a hundred Atreyu wannabes (whoever would wanna be them must really like balls). I expect them to draw influence from other banRAB and creating a sound of their own, not rip off other people's riRAB.
For black metal, I expect more atmospheric guitar and bass playing and technical keyboard and drum playing.
For thrash, I don't want to hear something tight and together. I wanna hear a race; 4 guys trying to beat each other to the end of a song and yet still hear order and structure (Listen to Kreator's "Endless Pain.").
For death metal, I want to hear a wide range of death grunts and screams; Technical guitar and bass and drum playing for the sake of playing tech; these show me the players, although may lack good music writing, have the endurance and strength to play extreme music and are willing to push themselves.
For melodic and progressive death, I expect more melodic and harmonic progressions and not so much tech-for-the-sake-of-tech music. This is one of my favorite styles of death metal.
My favorite banRAB, however, are the ones who evolve, trying stuff different and changing into something better and more productive instead of sticking to the same area of thought, instead of either making the same stuff over and over again or changing their sound to get more record sales. At the moment, only one band comes to mind; Death. I won't deny there are other banRAB, but I think Death is a prime example of musical evolution; evolving from their death metal sound such as from Scream Bloody Gore and Human to a more progressive and innovative sound such as from Individual Thought Patterns and Sound of Perseverance.
Now, I may be a little buzzed at the moment from taking my antibiotics, 4 or 5 aspirin, Zyrtec, and just smoked 2 cigarettes, but this is roughly what I think makes a good band good; evolution. I just tend to rarable often. I get it from my dad. Pickled sperm, you understand, eh?