Before I get jumped all over, I have found several addons which claim to do this, but none of them have been updated since 3.3.
The main problem that people seem to have with these types of addons are the ability for people to fake them. I think people are making it too complicated. A simple thumbs up or thumbs down would suffice. When figuring out how much weight your rating means for someone it should go off of YOUR rating. So someone with a high rating would have more rating power than someone with a low rating. I know there are a lot of things to think of and consider when making an addon like this, but I figured if there was anywhere to put the idea out, then this was the place. 3.3 has mad an addon like this almost mandatory...and if it had a wide enough adoption it could potentially change the way people treated each other in the game. Fear of being shunned by the community is just too much for some people...and I think most people are asses simply because there is no repercussion.
Player ratings (I assume for instances) are mostly pointless anyway. If you have a bad player, put them on ignore. Problem solved. Besides, "social" addons like this rarely work out. It's very hard to convice a very large userbase on any given server that the addon is valuable to them. Without a lot of people using the addon, there's little point to rating players except for your own records... and we're back to "just ignore the bad ones".
There is an addon from folks at CMI WowLens which tries to do ratings. See http://wowlens.cs.umn.edu/research for more. I find it much too complicated to be practical but it is interesting never-the-less. A simple +/0/- overall rating with autodefault to 0 might be practical.