Yes this is another "help me decide" post. I looked to see if these were allowed here, so I apologize ahead of time if they are not.
My Warlock was my original raider in BC, even then it was one of the better designed classes. I switched factions after BC, eventually brought it over when you could Xfer but I was stuck in a healing role for years (really until MoP). MoP was my "melee" expansion where I mained a rogue and a warrior. I felt like the rogue was always fairly solid - but to be honest the only spec that flowed well to me was assassination and compared to other classes the lack of variation was starting to show. When timeless isle came out it was a chance to switch to my warrior and I did. Loved it. I'm not sold on the new direction warriors are going in WoD so it's time for a new plan.
I'd prefer to play a "pure dps" class mainly because of spec varation. (I am done as a dedicated healer) - the only exception I have to this rule is a Shaman because they have both melee and ranged DPS, which in my mind is a greater variance than 3 ranged or 3 melee specs. I know in the previous sentence I said i was done healing, but the shaman would permit me to at least mop up LFR and dungeons faster queueing as a healer and facerolling it (but of course the need for that is temporary). A druid is the only class I don't have at least 85 - I have no interest in druids. The only way i'd play a druid would be balance with the glyph of stars (I hate animal forms).
I had my eye on Mages, but the mage forum here wasn't very encouraging - most of them mentioned "warlock" in every post. I hate the rune of power gameplay as well.
I have a hunter as well, but I just can't get into it. I'm not really a fan of the style.
I'll be doing "normal" raiding, unless "heroic" is the new "normal", then in that case heroic - LFR til the gear is useless etc. - Likely light pvp as well (ashran, BG's)
TL;DR - The warlock has the most "fleshed out" specs, specs within specs - different resources, cool tricks blah blah blah. So Warlock, Mage, or Shaman?