I think I'm the one exception to the level 70 demonology spec, as I'm affliction. I did level as demo, though, and only specced affliction when I dinged 63, so I'm very familiar with that build, too.
Aside from survivability, the one area that I'd give demo a serious nod over affliction is in single target damage, particularly in shorter fights. The +dam from demonic knowledge and master demonologist/demonic sacrifice really help in those situations. I have shadow mastery, which helps, but generally won't give me quite as much of a bonus. With both specs I had 11 points in destruction for quick(er) firing shadowbolts and shadowburn. In a quick single target fight, the demo 'lock pumping out his DK/MD enhanced shadowbolts and having his pet beat on the mob as well will almost always outdamage my SM shadowbolts and 'mana battery' pet sitting idle.
Where the affliction build truly shines is in efficiency. As I mentioned in the Shadow Priest discussion yesterday, I have so many tools to regain mana and health that I almost never have to drink. I have an Imp summoned 99.9% of the time, and leave him on passive, so he's always regaining mana at his non-combat rate, and I just Dark Pact mana from him whenever I start dropping low. If we're both low, then I lifetap and siphon life/drain life to bring my health back up.
The other advantage I have as affliction is in multi mob fights. In an instance, if the tank can hold aggro well enough, I can easily get DOTs up on 3 mobs at once. I have 5 DOTs, 3 of which are instant cast, and all of which hit significantly harder than the DOTs from a demo 'lock (who will only have 3 to choose from, 2 of which can be instant, if he's specced insta corruption). DOTting down 2-3 mobs in parallel is where my DPS starts to get scary. And, once again, in long boss fights, when other casters (including other locks) are having to worry about mana, if I can avoid aggro issues, I can literally continue casting forever by balancing my health and mana bars.
As far as levelling and grinding goes, affliction was so much easier for me than demo. As much as I liked the Felguard, there were times I had durability problems with him. If he had 2-3 mobs on him, he could drop in a hurry. It wasn't really a problem pre-BC, but some of the areas in Outlands can have insane respawn rates when they're heavily camped. There were several times that my pet was doing fine, then a repop would come unexpectedly, and he'd drop like a rock. I also had to worry about conserving his mana, as he ran out fairly quickly, even with a point or two in mana feed.
Once I went affliction, if I wasn't fighting at least 3 mobs, I felt like I wasn't making the most of my abilities. I'd DOT up 2-3 mobs, sometimes more, depending on where I was and how hard they hit. I'd usually lead off with UA, then CoA, Corr, and siphon life, then move on to the next mob. The great thing about that is that only UA required me to stop moving in order to cast. I could be running away from mob 1 and throw out my 3 instas on mobs 2 and 3, then hit an insta Howl of Terror (possibly the best 2 talent points I've ever spent), and all 3 mobs would usually be dead before fear wore off. Loot, dark pact, and repeat. With improved drain soul, every time I DS'd a mob as he died, I'd get 15% of my max mana back, so mana efficiency was insane. It was pretty normal for me to end one of those 3 mob pulls above 80% in both health and mana, so I could immediately go find another pull. Zero downtime while multi-mob grinding just isn't fair. DPS is very high, as well. Even though I was getting zero damage from my pet, this method of grinding increased my DPS significantly over what I was getting with demo. At levels 69 and 70, while solo grinding, I could generally sustain about 410-450 DPS when I was grinding using this method. I haven't done much of this since I've been boosting my +damage gear. That was at about +500-700 damage. I'm now about +920, and am sure that my DPS would be noticeably higher now.
Sorry for the wall of text, but as I'm sure you can see, this is an area I've put a lot of thought into. If you've got other questions, feel free to post here on the forums or send me a /w in game.
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