Yep really depends on the concert.
I generally try to dress light, I'd rather be cold outside than sweating buckets during the gig. I sometimes wear a tshirt of the band I'm seeing or a band similar or that I know the band likes.
Comfortable shoes are a must, converse just arent good enough-too thin.
Not too much in my pockets, only essentials that I know wont fall out.
I try to avoid wearing hats now, it pisses off people behind and its another thing to lose (or have nicked). Unless it's outside that is, if I'm at a festival I need hat to be standing outside in the sun all day.