in terms of materials, the battery back of the HTC thunderbolt had a nice soft touch semi rubber feel that i liked. if the entire back of the phone was made that way instead of just the battery cover that would have been a nice touch.
for me, i'd prefer a more durable shatter and scratch resistant screen before metal or exotic case materials. i still get the impression that most people use an aftermarket case, so the plastic vs metal angle never was important to me. especially having started off with blackberry back in the day. actually, the external carrier branding of phones is more annoying to me than case materials. the bendable screens that we've heard of sounds like it could be good progress in the screen area.