There is a small bug in the rating system.
Once somebody rates a certain thread, nobody else well be able to rate that tread.
It will just say: 'You have already rated this thread", eventho you never casted a rate to that certain thread.
It ain't that much of a biggy, but people might abuse it to put down some threads.
This post might help you out btw:
Can't rate threads that got already rated - vBulletin Community Forum - Projects