I love hard data over anecdotal evidence, and Cynwise crunches the numbers he is able to get:
Any thoughts on these? Has he missed something?
Warlocks from Vanilla to the end of BC (or even arguably LK) had this spec called "affliction".
It was centered on the idea of draining the life force of your foes to sustain yourself as you killed them (drain tanking)
In LK they seriously hurt this concept, and introduced death knights, which have a spec called unholy, which does damage with dots the same way, but doesn't require the self-heals due to plate.
The "coup de grace" happened in cataclysm, when they utterly gutted this central mechanic to warlocks and turned warlocks into wal-mart brand mages.
People got the hint, and went death knight or mage.
The sad part is it probably won't come back until blizzard cans GC, who positively hates the vanilla/BC version of affliction, and has architected this utter destruction of what was my favorite spec in the game.
I also say that "bring the player not the class" has NO PLACE in an RPG. RPG's are all about separate classes which each bring something to the table to make the group more powerful than the sum of its individual parts.
If you gut what makes classes unique (enhancement did mediocre dps but made warriors and rogues gods, shadow used to be the mana batteries, etc) then you may as well be playing world of halo-craft and give everyone a battle rifle.