The problem with the examples you mentioned is that the modding communities for those games are relatively small and self-policed. Also, the tools require a level of effort and know-how that would tend to discourage people from doing something simply to irritate people.

However, any player content system in WoW would have to be accessible enough to allow a significant amount of the playerbase to participate. That means something simple enough that it can be learned quickly and rapidly produce something. If it's quick and easy to produce something, trolls will be all over it, flooding the system with crap.

Not to mention gold sellers, who've proven they can corrupt any game system available to get their ads in there. (Remember the web addresses made of dead gnomes?) Plus, either the system would have to offer no rewards at all, (which would discourage most players from trying it) or said rewards would be gamed somehow: wow players have shown again and again that they can exploit any opening to get quick experience, gold, or gear.

What I'm saying is, (as Chris Metzen mentioned) the system would require a HUGE amount of policing. Think about Blizz's track record being able to enforce naming rules. Or ban racist, homophobic, sexist idiots from trade chat. How about enforcing their rules for RP servers?

The point is: the amount of resources it would take to make something like this work would be better spent on a thousand other things.