I had no idea this thread existed either! (Considering how old some of the posts are, I'm glad it has been resurrected.)
theleatherette, I went to Coachella this year and 2005. I always laugh when I see girls teetering around on the polo field in their Louboutins holding on to their bf's. I agree, it's fascinating seeing what people wear in 100 degree temperatures.
As has been mentioned, what to wear depends on the concert. If it's in a gorgeous old theatre and the seats are reserved, you can wear whatever you want. If it's general admission and you'll be standing for hours and hours, that's a different story.
For a moshpit/standing show, I think toe protection is crucial. Do not wear any kind of sandal or flip flop as some 300 pound guy WILL step on your feet. I have a few pairs of "gig shoes" - big rubber platform heels (so I can see), really comfortable, and I don't care if they get beat up. I never carry a handbag that I have to hold or wear on my arm or shoulder. I have a small one with a long strap I wear like a little messenger bag.
Personally, I think it's uncool to wear a t-shirt from the band you are seeing to the gig unless it's a really special one. If you see the Flaming Lips and you have a vintage shirt from the 1980's or a one-of-a-kind shirt, I think that's ok.