Okay, tanks and healers, let's be up front about something: we own LFD.
This came to mind while reading these posts on the forums about tanks queuing up for a dungeon solely to get the early quest item, then dropping the group as soon as they had it, forcing the group to wait to replace them. Tanks can do this because we have instant queues - if you want to tank a dungeon, all you have to do is queue up and you're in almost immediately. Healers sometimes have a bit of a wait, but usually not much of one, and can often have instant queues as well.
DPS? Well, DPS players (the most popular role, so we know it's somewhat self-inflicted) have to wait. On average, they have to wait up to an hour to get into a heroic. So if you're a DPS player queuing up for a heroic, it can be immensely frustrating to finally get that group you've wanted, zone in, help the tank kill the mobs he or she needs to get to that quest item, and then the tank drops the group, at the very least setting you back a half hour if not outright dooming your dungeon run. You did your part - you helped clear to the quest objective. And your reward is more delay.
Rygarius mentions that a solution is in the works (it's now live), that the end boss' death will be an objective of the quests, and I think it's a solid workaround. And the reaction of some players to this workaround is revealing in a way I find disturbing.