One way to help with the win/loss ratio rating is to have it only be positive, and neutral at worst. If you end up on terrible teams over and over, it won't put you into a hole. It'll take a while for the sample sizes to get big enough to evaluate accurately, but within a few months you should start seeing a separation between the 5:0 team baiters and the 1:1 sacrifice players.
The classes aren't that important if you're not evaluating based on dick-waving stats. I'd rather have a super-smart player with terrible aim as my sniper than a dead-eye sharpshooting numbskull.