Well,
They lack key spells, and second talent tree talents, ie; you only get enough talent points at 70 to spec into your end-tier talent (Haunt, metamorph, Chaos bolt).
Key abilities being, Demonic portal, Demon soul, Shadowflame, Fel flame for mobility, mastery, and the lesser, Seed of corruption moved to 72, leaving Affiliction with almost no AoE.
Plus with how abilitys scale, Mages/rogues just destroy most warlocks, so most people opt to play Rogues, Priests and/or Mages.
As for single-target PvE; Warlocks are great, rotations are mostly intact, (Unlike, say, hunters, lack of cobra shot for Sv/Bm) and so on, good fun.
Least, that's why i think nobody plays them in this bracket.
And sorry for the small wall-o-text.