For today, we'll take a look at my personal favorite, the Felhunter. I love the Felhunter because its cute, scaly, and eats my opponent's buffs and locks them out of magical schools. Out of all the demons -- with the exception of the Succubus -- Warlocks get the most benefit from micromanaging a Felhunter. This demon changes the most significantly in Wrath owing to a few things. The bad news first: currently in Beta, Felhunters no longer confer Paranoia, so in Wrath you'll actually get stun-locked by surprise instead of seeing it coming and not being able to do anything about it anyway. It also loses Tainted Blood, which won't be missed because a Felhunter with a Rogue or Warrior on it is going to die, -475 attack power notwithstanding.
Now for the good news. Felhunters now have a passive, raid-wide buff called Fel Intelligence, which increases party and raid members' Intellect by 48 and Spirit by 64. Neither buff is as strong as the individual buffs of Arcane Brilliance and Divine Spirit, and do not stack with them. On the other hand, it's an excellent group buff that helps in parties and when solo, although Warlocks have yet to see more gains from Spirit aside from better Life Taps. [EDIT: Oops, I forgot about Fel Armor. Cut me some slack, though! It's not like Warlocks ever pursued Spirit before...] This means that Felhunters will actually be more useful in PvE while retaining PvP viability thanks to Devour Magic and Spell Lock.
Felhunters also gain a new offensive ability called Shadow Bite, an attack which scales with the number of DoTs you have on the target. Get the picture yet? With the new direction Blizzard is taking in Wrath, the Felhunter is shaping up to be the Affliction Warlock's pet. This is reaffirmed by the Improved Felhunter talent which is deep into the Affliction tree. This means that aside from the group buff, Affliction Warlocks get a pet that can do DPS that scales with the Warlock's playing style. While this is awesome in itself and will finally see Felhunters brought out in instances, the greater applications lie elsewhere in the Affliction tree.
You see, Improved Felhunter guarantees a constant stream of mana for the Felhunter, making it an ideal target for the new and improved Dark Pact, which is an instant mana gain of at least 1400 that scales with spell damage. I haven't done the numbers, but Felhunters are now competitive with Imps as mana batteries for Dark Pact Warlocks. In fact, Felhunters get the edge because they can -- or actually need to -- attack while acting as mana batteries whereas Imps must be set to passive. All demons also receive the passive Avoidance, which increases pet survivability in raids.
Affliction Warlocks can opt to take points in Demonology for Fel Vitality and the more accessible Soul Link, allowing the Warlock to go SL/SL while going deeper into Affliction for Unstable Affliction and Haunt at the expense of Demonic Resilience. The itemization for Wrath seems to hurt Warlocks because Stamina for cloth will be somewhat homogenized, meaning the days of Warlocks having life equal to Warriors may be numbered.
In PvP, this means less Life Taps and more Dark Pacts, as well as more judicious use of Devour Magic. There's also a moderate bonus for keeping the pet on a target with DoTs and not merely a Fire-and-Forget pet sent to a caster. In PvE, this means that players will actually see Warlocks summoning a Felhunter in an instance and not get laughed out for using "the PvP pet". Slap on Glyph of Felhunter for good measure if you want, since a Felhunter's DPS actually scales. Good times.