Superman is always willing to risk his life, and he is vulnerable to kryptonite (green or otherwise), magic, 5th dimensional imps, power drainers (the parasite, anyone with red sunlight duplicators), stronger foes (Doomsday, Mageddon (he needed batman to pull him out of that one - Morrison's run on JLA)), etc. That doesn't count the threats to his friends and family, or to his reputation (the 60's *choke* *sniff* soap opera silly stories). It *never* stopped him from trying to save others. Sounds pretty heroic to me.
My version of hero is "going above and beyond the call of duty". Since Superman is not officially a cop or anything, he has no duty beyond that of an American Citizen (well, a reporter has some duties, so he has those). Thus any time he does anything beyond what is required of an American Citizen (or reporter) he is being heroic, and if he is heroic consistently, then he is a hero. Since he also happens to have super-powers, he is called a Super-Hero.