Agree with some of the above, Cata for healers because of mana management, actually having to think about your rotation, spells with little mana cost being useful, calculating the advantages of different gearsets for more mana or spirit etc.
MoP killed hpal for me, I hated the Holy Shock patch and the new fixed mana bar.
MoP was fun with elecrocuting people on the run on shammy though. Enjoyed that at LM, plus thunderpunting people is always great. Thunderpunt and Dominate Mind most enjoyable abilities at 20.
DR / removal and the sacrilegious nerf to Whiskey and Rum turned WoD into a bleak and barren wasteland of desolation for me (i.e. I haven't played enough to give a valid opinion).