I had to leave a DF party twice because of 2 things:

1) 1 party had a great tank but a CNJ that didn't heal at all ( only healed himself if he was hurting >.>)
2) Another party had a Great healer, but since tank didn't use flash at all, all the mobs went to the mages and they both died ( then the tank died as well)

Both occasions I have repeated ask those people to please use "Flash" or "Cure the tank" in the nicest way possible, they refused to listen and died 6 times in each party.

Then I left the party.

I always give EVERY PARTY a fair shake, but if I don;t see progression of some sort at all, that's when I leave.