Demonology. I play destro and afflic to an acceptable level but I have tried many times to go demo for pvp and/or pve but I just don't get it as a spec. Don't know why, even at the start of MoP when it was supposedly OP I couldn't make it work, couldn't do anything but get ran over in PVP when every other demo lock was godlike.
I also have not had a healing spec since Wrath. I could probably do it if I tried, but no interest.