Right now there's no punishment at all (other than not getting the CCs/prizes from matches you leave). Their long-term strategy is to have the matchmaking match leavers with each other, but I don't think the player base is big enough yet for that to have any effect.
I certainly wouldn't mind if they gave leavers a slight CC penalty for the next 3-5 games, but I think Uber is worried about alienating paying customers.
I still have a love-hate relationship with this game... I've been matchmaking with a friend, and we rarely win games anymore (even though the two of us are consistently top of our team, usually by a decent margin). Neither of us can tell if we're doing something wrong, or just getting constantly screwed by the team balance.