When Blizzard started work on the Warlock class for Wrath of the Lich King, I imagine one of their goals was to make the underrated Affliction tree viable for raiding. Unstable Affliction was a great spell for PvP, but anyone specced deep enough for it wouldn't have Soul Link and thus were practically free kills in the Arena format. In raids, it was a wasted spell because most mobs and bosses don't dispel. It also had a cast time that ate into the Warlock's spell cycle. It was a great spell on paper, but lost a lot of luster in practice.
Now along comes Haunt, the 51-point talent in the Affliction tree. It's a direct damage spell that applies a 12 second debuff that increases the damage of the Warlock's DoTs on the target by 20%. In addition, at the end of the debuff's duration or if it's dispelled, it heals the Warlock for 20% of the damage it dealt. It's a nice effect but the amount healed is rather negligible so I hope that aspect of the spell gets tweaked some more before it goes live.
The spell deals direct damage, applies a debuff, and heals? I don't have to tell you how much that rocks. But I will anyway -- it rocks. The heal is underwhelming, yes, but the debuff is just plain awesome. It's essentially a permanent 20% DPS increase with a kick. Furthermore, the reworked Shadow Embrace applies an additional debuff for Shadow DoTs, pushing Affliction Warlocks to toss Haunt and maybe Shadow Bolt as preparatory spells and a cycle of damage-over-time spells, then rinse, repeat. Over a lengthy fight, Haunt and two stacks of Shadow Embrace can be permanently on the target for a huge 30% damage bonus to Shadow DoTs (20% for Fire). Actually, make that 40% for Shadow spells with Shadow Mastery. A Haunt crit that deals 40% more damage? Delicious.
The fantastic thing about the Affliction tree -- bloated as it is -- is that everything clicks together. The spell haste from Eradication is critical to putting out faster Haunts, Shadow Bolts, and a reduced GCD for DoTs, while Pandemic adds a burst element to a suddenly bursty Affliction tree. Affliction Warlocks can pursue crit gear and finally, properly have their DPS scale. Affliction has always shone best over longer duration fights, but Haunt gives Affliction locks a way to deal burst DPS for shorter encounters that doesn't penalize DoT application as Shadow Bolt sometimes does. With another 1.5 second cast spell in Affliction's arsenal, the only thing that needs work now is Unstable Affliction, which should really be instant cast to properly fit into the DPS spell cycle. Haunt is one of the best spells that Warlocks have right now and is easily my favorite one in the Beta. Now where'd I put all my crit gear...