I wouldnt worry too much.
I've been to loads of concerts, first one was stone sour and murderdolls back in 2003 teh day before my 14th birthday, and i got pretty banged up from that, just a few brusies though.
I went to the sunday at download last year, was in the pit for slipknot and machine head, and again, i got pretty beat up, but it's nothing too serious...and i think machine heads pits are meant to be the worst out of the more mainstream metal bands, correct me if I'm wrong though.
Saw fozzy and bowling for soup, but there not the sort of gigs that your going to get rough crowds at.
Going to download again this week, all weekend (well once i've done my english exam on the friday) Slipknot, Slayer, Sabbath, System, Mudvayne, Megadeth, Napalm Death etc. I'm not exactly big, 5"8 i think, and i only weigh 8 anda half stone. So i wouldnt expect anything worse than a few bruises
As far as best and worst go...drowning pool and slipknot at download last year were by far the best bands i've seen live, worst would be korn at download, they were abysmal.