There's a problem with mass-quitting in LFR right now because people are being incentivized to only kill one or two bosses. Players pick up their weekly Emerald Nightmare or Nighthold cache quest that requires them to kill a boss in that raid instance. I appreciate that even if you're a mythic or heroic raider you can run your weekly cache missions in LFR, it makes life a lot easier. I appreciate that the Devs tried to add some flavor and variety to the quests by mixing up which bosses need to be killed, but the end result right now is a huge mess for people running LFR. 75% of the people in a given LFR instance are bailing after the quest objective boss.
As an example, the EN cache quest this week (for me at least) was to kill Il'gynoth. IllyG is the 2nd boss in the Darkbough wing of LFR Emerald Nightmare. I queued up three times today before I got a fresh run. The first two times I waited the 10-20min for the queue to top, then joined a group that was already past the first two bosses and just getting to the third, Elerethe. The first time I killed Elerethe for the AP and the completion gold. The second time I quit and re-queued later for the same reason most people do. Finally on the 3rd attempt I got a fresh run. I didn't quit after IllyG, but I really wanted to along with the other 2/3's to 3/4's of the group who did bail. And as we're making our way to do trash for Elerethe, in comes a whole new fresh group of players, many of them complaining about a stale run.
I thought we fixed this same issue of mass quitting in Warlords by requiring dungeon quests for the Inn to not only pick up a certain item, but kill the last boss of the dungeon. It's a quick fix. I still queue up for LFR for the AP and to work on my weak auras and try out new talents and gear.