[MunchkinMan]

Warriors have a permanent +1 that costs nothing: They win ties.

I am going to suggest that arguing about balance is of no use. I will contend that most people see a lack balance where I see lack of experience. In truth, you described a situation where everyone fed the Thief and now the class is suddenly unbalanced. The problem is, people kept the Thief stocked with cards. That's, as progmode said, a group dynamic issue, not a balance issu. You keep the Thief in check by not giving him cards.