Its been awhile since i have tried a warlock and wanna have some fun lvling one when i got nothing else to do in WoW. Im just unsure as to which spec i want to lvl with
Im aiming for a spec that got good AoE but also is a good spec "on the go" which means that i dont have to sit around waiting for ages for my cast bar to complete a spell
Demo seems fun but im not a big fan of the demon form which i feel is a huge part of the spec which seems like it will be a waste taking that spec
Affliction also seems fun with the DoTs but seems to lag good AoE stuff before Seed of Corruption which is first learnt at lvl 60 (unless multi dotting different mobs is a good AoE and "On the go" spec)
Destruction seems a bit boring with all its casting time abilities and seems to rely heavy on that which isnt really my thing.
Multidotting is good, but you have to use your voidwalker to tank for you when doing that. And since Blueberry can't taunt off of you as much as you'd like, you can't just pull gigantic packs of mobs and hope to survive.
If you're wanting to AoE grind, then Demo with the Glyph of Demon Hunting is your best option. Strong damage, strong AoE, ridiculous survivability. Also because you'll permanently use Dark Apotheosis form with that, you'll never be using your Meta form. So you wouldn't need to worry about that.
Pretty much what the previous poster said, though don't underestimate affliction for "aoe leveling," particularly when you get access to the Grimoire of Supremacy. The voidlord is an excellent pocket tank, and will pick up things that are attacking you pretty quickly. With 4 to 5 targets - 4 soulburn + soulswaps and one hard soulswap to the 5th target it works pretty well.
Beyond that you're looking at demo. For large packs of mobs it's hard to beat demo, particularly if you use the Dark Apotheosis glyph. Though that glyph reduces your damage a bit, you can pop immolation aura, hand of gul'dan harvest life, and your choice of Felguard / Wrathguard to aoe things down pretty fast. Fury Ward, Unending Resolve, and Dark Bargain will keep you alive if it gets hairy.
I always play demo, but i'm sorry while dark apotheosis may be fun sometimes, it really sucks.
Go in demon form with 0 fury for example and pop wrathguard bladestorm it already does mad aoe.
Also for lvling, with the glyph you get 20% hp back for every kill made while being in demon form, enough healing right there
The Voidwalker and Fel Guard both have passive abilities that increase their threat by a very high amount. They cause enough TPS to take threat off tanks while leveling with this ability turned on (Threatening Presence I think its called). If youre good at pet management any Lock spec will be great for AoE leveling. You can make a macro for one of your spells, Unstable Affliction for example, and tie /petattack into it. Then the pet will automatically attack anything youre doing a full round of DoTs on. Or macro /petattack into your Soul Swap. Any lock spec can be great for multiple mob grinding using this technique, though Affliction and Demonology are the best choices, with Demonology being slightly better because of the survivability, but in my opinion even with the Demon Hunting glyph the difference wont be much. Personally Demonology seems more "easy mode" for this, whereas Affliction would seem more challenging and rewarding.