Devouring Plague has gone from the unique racial spell of Forsaken Priests to a baseline ability for all Priests, changing a little on the way. The cooldown on it is much, much less than what it was, becoming part of the regular Shadow Priest rotation. Its cooldown makes it a little awkward to use while solo grinding, but it's a bread and butter DoT now so get used to it! Using the Shadow Priest soloing tactic of spreading DoTs across multiple mobs, you'll be able to drop Devouring Plague on one target but not the others. And you should, because the healing is nice and so is the damage, even if it's not one of our heavy hitters.
Vampiric Touch has been part of our repertoire for awhile now, but it's a bit... different now. Its mana regeneration component was very severely nerfed and replaced with Replenishment, but there are other changes that were made to the spell as well. The coefficient on the spell has been vastly increased, making it a priority spell even with the nerfed mana regeneration. Shadow Word: Pain used to be our powerhouse DoT, but no longer. Vampiric Touch is doing far, far more damage than Shadow Word: Pain, almost doing as much damage as Pain and Plague combined. If this wasn't the first spell you cast before, it should be now.
Shadowform has been the defining ability for a Shadow Priest. Not only does it completely change the visuals of the Priest, it turns the Priest playstyle on its head and always has. You're not a Shadow Priest unless you have Shadowform. That simple.
Shadowform has become even more powerful in Wrath of the Lich King, especially if you drop two talent points into improving it. On top of all of its previous effects, Shadowform at its base now reduces threat generated by 30%, and increases damage dealt by your DoTs by a percentage equal to your crit chance. If you improve it you get added benefits, but the base Shadowform is really good enough as it is for most things.
Mind Flay 2.0
How we use this spell hasn't changed at all, but the spell most certainly has. In fact, the spell was completely rebuilt for Wrath of the Lich King. Like channeled spells such as Arcane Missiles, each tick of Mind Flay can crit now. On top of that, it looks totally awesome.
One of the things exciting me most about playing a Shadow Priest in Wrath is the fact that we finally get a Shadow-based AOE spell: Mind Sear. Its a little clunky in its use, being a channeled AOE centered on a mob, but the damage on it isn't shameful. It's a powerful spell once you get used to using it, and the pushback changes that were implemented in Wrath (along with Improved Shadowform) actually makes it feasible to use while grinding.
Obviously we have other things at our disposal. Mind Blast, Shadow Word: Death, Vampiric Embrace, all that stuff, but they haven't changed in ways the other spells have. Vampiric Embrace was hit with a party/raid nerf, but its personal healing stays the same, so for leveling purposes it hardly matters.
While our mana regeneration from Vampiric Touch has been hit with a nerf, our grinding capabilities are as good as they've ever been as far as mana goes. Spirit Tap, Improved spirit Tap, lower Shadowfiend cooldown via Veiled Shadows, and Dispersion on top of all of that, we have plenty of mana. Lay all of our brand new DPS on top of that, and we're on a fast track to 80.