In an analysis of statistics collected by PlayOn, they said:
players who choose to be warlocks are probably different from players who choose to be priests
I find that sort of comment quite interesting, as my two level 60 characters are a warlock and a priest (with the former being the one I view as my primary character). When selecting a second class to play, I intentionally chose something quite different from my warlock, in order to maximize the novelty in my play experience.
My priest is specifically tuned to be a support mechanism for groups, as opposed to a strong soloer or PvP combatant. Despite this, I find myself playing my warlock exclusively in end-game raids. Part of this is because, while I enjoy playing my priest, a warlock in a raid has a very low-stress job. Success or failure of the group as a whole is not dependent on your split-second reactions, unlike a priest. A contributing factor is the relative class distribution. As uncommon and valuable as priests are, warlocks are even more so, especially in the Alliance. I have been the sole warlock in a 40-person Molten Core raid, which is quite beneficial to me from an item-acquisition perspective, even should my play style preferences not be an issue.