There are 2 times I leave content:
1) The tank left and we were unable to get a new one within 15 minutes.
2) I join a crumbling party in progress, which is supposed to be 8 men, but only 3 are left, and every new person joining instantly quits.

Otherwise I either try my damndest to clear, and help the people struggling, or (if they are not showing any sign of progress) I initiate a vote kick.

I have NEVER eaten the 30 minute penalty for leaving first.