Being able to play Demonology in a competetive environment. Yes, demonology was very strong during 5.2 and 5.3 and felt kind of gimmicky with Unerring Vision of Lei Shen. But boy did I have a lot of fun playing Demonology regardless. I think Blizzard nailed Demonology with MoP, but unfortunately it's only been viable for less than half of the expansion. Right now it's among 1 of the least played specs in hc SoO and for good reason.
Affliction and Destruction are just way ahead in terms of damage compared to Demo now. Affliction is as good as it has ever been and Destruction was severely buffed with 5.4. Affliction to me, is just outright boring. It certainly is a good spec, and has been for as long as I remember, but I feel that Affliction is nothing but looking at the timer of your DoTs and refresh them based on remaining time and procs.
Destruction to me has always been the easy spec. It was in a fairly good spot back in 5.2. It was the "newbie" friendly spec which dealt decent amount of damage for little effort. But now, it deals insane amounts of damage for little effort. It is pretty much your go to spec for warlocks right now, and the lack of any form of complexity in the spec just kills it for me.
Since we're not going to see Demo viable in 5.4, I hope they bring back Demonology in competetive play for WoD. It's so sad to see such a fun spec go to waste imo.