Which spec do I prefer, and why?
Right now I've been playing a lot of Affliction, both in PvP and PvE. I really like Demo, but I don't like that either a) I'm completely missing something, b) I have to be in melee range to get the full benefit of my Metamorph, or c) that the tanks keep dragging my target out of the Hand of Gul'dan. Seriously, that is a shadow patch you CAN stand in - though I do admire your survival instincts. Destro really feels complicated and clunky. So much of your DPS depends on keeping up a buff that 'should' be proccing from your demon, but I've found I have to hard-cast more often than not. I'm actually pretty sure that my obsession with getting my three hasted casts off is what's killing my DPS as Destro, but who konws.
For whatever reason though, Affliction just feels more like a Warlock spec than the other two. In all honesty though they are so very similar that it's kind of funny. You have three DoTs you want to keep up at all times (Corruption, your Bane, and either Unstable Affliction or Immolate), a Curse (usually Elements), one or two spells with CDs, and a filler. In this regard, I'd rate the specs (in terms of difficulty) as Demo>Aff>Destro - but that's just me.
I am glad though that they aren't just different versions of each other, and that there are slightly different aspects to each spec. I guess the one thing I don't understand though, is why does Haunt refresh Corruption instead of UA? Goes back to that slightly different feel again.