It would take a special community of people to rate the players. A community that would soon be over whelmed by the task of rating so many players.

The game would definitely have to have something built in like this:

Per account you can only rate a person Up, Down, or not at all. You can only rate them once per account. (what's the mod that shares your friends across alts? Yeah, something like that) You can't bump them up multiple times or down multiple times. So if they're rated down by you and they do something decent, you can rate them neutral/eventually positive.

Built in will have to be some kind of formula that says, "ok. this guy reported 15 people in 5 minutes." and negatively effect the reporter's rating because it's not often that you come across everyone that's absolutely worthy of a bad comment. In most cases, 3 or 4 people can take care of all the ignorance for the rest of the people that may be accompanying them in which case a guild rating could be considered (though that could get ugly).

Just something to think about. Most people in this community are pretty insightful, I'm sure Wowinsider folks could come up with something along these lines.