Leme explain it to you. Since Cata Affliction has been the MOST viable spec for a warlock in PVE and PVP for various reasons. I've been playing a warlock since vanilla and have two lvl 85s (prior to this bot) and have an extensive knowledge of the class in every aspect.
Majority of gear has stats that work perfectly with priorities
Multi Dotting for multi target fights increase DPS
powerful finisher (<25% health)
Simplistic rotation with minor variances
Only recently with the buff of Demo as of 4.2 has it even been possible to play that particular spec. Due to the fact that mastery on gear has been increased as well as the buff given from mastery (was nerfed partially in 4.3 because the gain was too high with Doomguard and Inferal) but the overall necessity for haste remains the same. Also with the recent buff to Destro as of 4.3 it too has become more viable. Granted in comparison of Afflic and Demo in theory crafting Destro pulls only very slightly ahead (approx. 200 dps in all heroic gear) they are still very equal with being viable. Most warlocks will tell you to pick what you like. Sadly that is where they couldnt be more wrong...More or less each fight specifically demands one spec over another in order to pull the highest DPS. They are doable as any of the three but each will have its own advantage depending on the style of the fight. Such as a fight like Warmaster Blackhorn. Demo will have a nice advantage over the others in cool down rotation burst dps but will lack in the Multi Target Dmg. Where as Destro with its Bane of Havoc can do alright dps with damaging an additional target for a % of the dmg done to the main target but seeing as it can only be cast on one mob its over all effectiveness is weakened. On this particular fight Afflic would be the best as long as the warlock is adept enough to ensure that their DOTs are on all possible targets while rotating well enough to focus their main target down effectively to not lose too much DPS.
Now for a fight such as Warlord Zon'ozz Demo would most likely win due to its vast similarity to a Patchwerk style fight with the least amount of movement and change to rotation during the course of the fight. In some or all of these cases with concern to Dragon Soul fights only Destro and Afflic will prove to be about the same overall aside from certain other variables as i mentioned above with the Warmaster encounter. The other important point to consider is how gearing up with each stat priority of each spec will ultimately change the overall complexity and viability until a certain point of gear can be achieved.
Demo 1: Int > Hit to cap > Haste (1007 rating) > Mastery > Crit
Demo 2: Int > Hit to cap > Haste (1573 rating) > Mastery > Crit
Demo 3: Int > Hit to cap > Haste (1993 rating) > Mastery > Crit
** Also note that playing a demo warlock there are two specs one specifically gauged to multi dotting (SB) and another more Single Target (Incin)**
Destro: Int > Hit to cap > Haste (2681 rating *403-416 ilvl gear)) > Mastery > Crit > Haste (Mastery gives buff all fire dmg and since Destro is primarily fire it is mostly better than Crit. However the increase per point is low enough that mastery will not be prioritised over crit or haste until much higher gear levels.)
Afflic: Int > Hit to cap > Haste > Crit > Mastery (mastery is still not strong enough for an Affliction warlock to be viable enough to replace crit)
With regards to haste there are only thresholds for certain DOT spells due to at each level of haste that is when that particular DOT gains another tick.
All Raid buffed:
Unstable Affliction
Extra Ticks
Goblin
1 9.9% 218
90
2 30% 2589
2437
3 49.9% 4951
4776
4 70% 7326
7127
Our other DoTs also benefit from gaining extra ticks. The haste thresholds for our other DoTs are below:
Corruption
Extra Ticks
Goblin
1 8.3% 21
0
2 25% 1993
1846
3 41.7% 3970
3804
4 58.4% 5945
5745
Shadowflame
Extra Ticks
Goblin
1 16.7% 1007
870
2 50% 4954
4778
3 83.4% 8911
8696
4 116.6% 12838
12584
**Some examples copied from Elitistjerks (
Elitist Jerks