As far as I can tell, leavers are not a problem at all in the beta. Leaves are bad to have on your record, for one thing. For another, a game is considered "abandoned" once one player has left and not returned after a short period of time (which exists for accidental disconnects). Abandoned games can be left without penalty by the rest of the players, the game is basically considered to be over the moment it is abandoned with no winner and only the leaver as a loser.
So if you do have a game ruined by a leaver, you can just leave the game yourself and find a new one, you aren't forced to play out a stupid 4v5 game. It also won't count as a win or a loss.
After beta, I am not sure if things will be as good, as I am sure part of the positive atmosphere right now is due to beta invites being rare and valuable. If valve does release dota2 as a free to play game people may have troll accounts and the annoyances will increase, but maybe they have a solution for that- for example only free to play if you already have $100 of games on your steam account, so you can't just create a junk account to troll on.