There's no single thing about this movie that ruins it, it's everything put together. One never gets the feeling that they're anywhere but on a Hollywood set - you're not transported back in time to an exotic, lawless time. The characters have no depth at all - instead of a feared assasin, The Rock comes off as a really nice guy, good with children, wouldn't hurt a fly. Forced to do battle, but he certainly wouldn't otherwise. And the whole thing is just so silly and cheesy. There's a comic relief character, but there's absolutely no need for him as there's nothing to "relieve" us from - the whole movie is little more than comic relief. It's just so unrealistic - it's obviously made for "the whole family", but that just points out it's major flaw - making a movie set in a time where people hacked each other up with swords to settle disputes, but do it in a way that the kiddies will be able to watch and think it's fun. It comes off as extremely watered down. As if it's a pilot for a Saturday morning cartoon or something.
Needless to say, you don't care about any of these generic Hollywood do-gooders, and there's never any doubt that they'll all come out okay in the end. Even the action sequences leave you with the feeling that that you're watching the World Wrestling Federation, where one guy might lose the fight, but he'll be back in the next episode good as new. It's not like anyone's getting gruesomely dismembered like in a real barbarian movie.
Basically, if you want to see an adventure movie, but it has to be something the kids can watch as well, you might get stuck with this. If you're looking for a real, involving adventure, get something else.