The Rock Band games have had a funny effect on the way I describe my taste in music. Before, I was content to say "I like metal." Now, I feel compelled to say "I like metal, but not the metal Harmonix keeps picking" instead.
My issue with the metal that routinely shows up on RBN is the overwhelming tendency for the vocals to be stupidly easy (all talkies or something close to it) while everything else is really, really hard. I play expert vocals and expert guitar/bass. All-talky songs are boring as hell and I find a lot of devil-tier guitar songs too difficult to be fun, so it's a lose-lose situation for me.
My DLC wishlist is loaded with metal bands whose vocalists sing and whose music does not sacrifice melody for abrasiveness. I'm talking about bands like Amorphis, Blind Guardian, and Paradise Lost, whose music seems destined to remain unplayable in Rock Band forever.
Oh, and I totally agree about the Jonathan Coulton overload. I have purchased and enjoyed some of his RBN songs, but he's had so friggin' many while so many other songs remain out of the PS3's reach that I've stopped buying them on sheer principle.