Assuming that you aren't using a proxy, and assuming the operator of the site is technically competent, they may be able to locate you geographically (roughly speaking), identify your ISP and, of course, if you make serious trouble they can hand over your info to the Man.
What a haxor could do with your IP addy is still a little unclear to me, but generally speaking I don't think it's something you want to hand out like candy. I think it could potential open you up to hacking attempts.