Without a doubt, this has to be my favorite warlock spec. I used it both for leveling my warlock and gearing as I ran heroics. Naturally, since I love it so much, I cannot bare to part from it as the raids have started. Do not ask or tell me whether it is the best spec – it is not the worst and the rest depends so much on what encounter(s) you are running. The debate between affliction and destruction is ongoing and so far, I have not seen a conclusion to it. Especially since it has to be restarted every time there is a patch.
But since the spec has worked so well for me, I figured I would share it with the community.
Leveling affliction is fairly straight forward. You need not worry much about gear, but the benefits of the warlock class are slow to manifest. This is why most people do not spec for affliction at low levels and truth be told, you will depend on your demon a lot during the first stretches of your career. The voidwalker is your most priced asset until level 30, when you can finally re-spec and get on with affliction. At around level 40 you should change to felhunter and you will notice why; your minion simply cannot hold on to threat anymore and the demon dog does a lot more damage than the dark bubbleman.
As you level, make sure you understand only one concept about gear; spell power is priceless. It is the only thing you should concern yourself with, all the way up to level 80. Your goal is to get 100 spell power by level 32. This doubles every 15 levels or so. That means, you should aim for 200 spell power by 45, 400 by 60, and 800 at 75.
This might sound a lot but is actually extremely easy to accomplish with nothing else except quest items. The only time I ventured into dungeons, was at level 60, to Hellfire Citadel. The trick is to always try do group and dungeon quests, if you find you are lacking spell power. The blue items are invaluable and once you reach those limits, you will steamroll the quests.
Other than that, playing Affliction is very simple. Let your dots and pet do the work for you, replenishing your mana and health as needed with dark pact, life tap, drain life / soul. You will find that you can easily run from mob to mob, having anything between 2-5 on you at any given time. You should carry food and water with you, but you have almost never a need to use them. Let me put it this way; each time a new type of water came available, I would buy a stack of 20 of it and usually have over 10 left before I reach a new type again. Instead, mana and health potions can be extremely valuable, in cases on emergency when you accidentally pull too much.
I would actually divide this into two more categories, heroics and raiding. You are at your weekest, in heroics because gear has advanced to a point where your group will simply plow throw both mobs and the bosses. Why is this a bad thing? Because affliction needs a more lengthy fight to bring its dps to up. Dropping Loken in 51 seconds does not give your dots a lot of time to do their job.
Make sure you always have your spellstone, fel armor, and life tap on. If the fight has a lot of aoe damage going on, refresh your life tap with dark pact instead but keep these buffs up, or your dps will be second grade, no matter what you do.
On weaker mobs (meaning anything outside the Frozen Throne) I end up spamming seed of corruption and shadow bolt. If the group is 4+, I hit a couple of them with the seed and finish with rain of fire.
For boss encounters, affliction is maybe the easiest spec. As always, do not think about rotation, think about priorities, which is to keep your dots up and fill in with shadow bolt when you have nothing else to do.
Start with Haunt, before moving to CoE and Corruption. After that, cast UA and then a quick Shadow Bolt. From there on, your main concerns are to refresh you haunt and UA, whenever they are about to expire. The main skill is to not clip your dots and only refresh them at the very last moment when they are about to end. This requires you to constantly monitor the dots and the best tool for that is an addon. I personally like ForteXorcist, which is a good tool regardless, regarding your warlock.
Other tricks are how to move and switch targets, like in the Emalon fight, without losing your dots on the boss, or stop to aoe adds on XT-002, again without losing dps on the boss. However regardless, it is always about maintaining your dots. Shadow bolts make up a lot of your dps in between, but if you dots fall off, even for a few moments, it usually shows in in the statistics later on. So some practice is required.
So far, the best spec I have found is the 56/0/15.
For more leveling details, suggest you have a look at this leveling guide. It will outline most of what I said and quite a bit more as well.