This is itself the problem. There isn't any defined purpose for the up/down arrow, so people just interpret it with their own vague standards and act accordingly. Everybody will be able to make a case for their up/down votes and everybody will be right, but will also reveal very little, because you simply can't tell what the vote was on.

If you gather a bunch of crazies and give them voting buttons, the result will still be crazy. If you take away voting buttons from sensible people, the discussion will still be sensible.

The problem isn't that the voting promotes group-think. It's the entire premise of voting that is skewed. Once every while you can find a comment here, downvoted to oblivion, simply for stating an unpopular opinion. Before the issue of group-think comes into mind, you'd have to wonder what the down-voters are thinking, and further, how the way they think influences the overall discussion here.