...and real world costs of time (dd DF wait time is ridiculous still).
To be fair, that isn't a DF issue so much as it is an issue of player choice.
There are 2 tank jobs (PLD and WAR), 2 healer jobs (WHM and SCH), and 5 DD jobs (BRD, SMN, DRG, MNK, BLM).
Since DF breakdown consists of 2 Tank + 2 Healer + 4 DD, that means if all things were equal, there would be almost no wait for tanks and healers, and a short wait for DD (by virtue of the fact that they have five classes attempting to fit into four spots).
But, as is the case in many MMO's, most people want to deal the big damage. Tanking is the most unpopular role overall, with healing a close second. DD's are a dime a dozen however, which is why I can get on my WAR and get a DF pop almost instantly, but my DRG takes anywhere from 5-60 minutes, with the tank almost always being the slot to fill last.
SMN has an advantage in this, in that they are also a SCH (assuming they have done the quests, which are trivial for a 50 SMN to accomplish). There is some disparity in gear usage as the levels tick past 40, but I know of more than one SMN who opts to run SCH when it comes time for Myth and Phil tome farming, simply because the Q's are faster.
Having said that, yes, it sucks when some ******* tank comes along and ruins the DF run of anyone's instance. DD can be replaced (and often are). But the DF itself is not the problem. It would be wonderful if SE implemented some sort of rating system however, where you rate someone's performance in a dungeon (something simple, such as a 1-5 rating scale on three questions). If someone leaves early, a new option pops up allowing you and your group to label them a "leaver." If a person accrues enough "leaver" score in, say, the course of 2-3 days, then they are punished in some way, such as being unable to access any DF content (or BC too for that matter) for a period of 24-72 hours. They can still play the game, do FATE's, craft or what have you, but they can't run into an instance and troll the crap out of people.