1. The death of Shadow Bolt spam
My favorite part of 2008 has to be the death of the mind-numbing Shadow Bolt spam build. The introduction of two new tiers of talents and a near-complete overhaul of the trees killed the most popular raiding spec that consisted of killing off the pet -- a sinister but arguably inane technique -- and pressing one button. In Wrath of the Lich King, Warlocks aren't forced to spec just one way in order to be competitive. Sure, some builds might be more efficient than others, but pretty much all three trees are raid-viable. Alright... two trees. Demonology needs a little work.
2. The death of SL/SL
Of course, just as the one viable raid spec died, so did the dominant PvP spec. SL/SL, or the Siphon Life / Soul Link combo, simply isn't good enough anymore. Actually, with the homogenization of cloth gear, Warlock Stamina took a nosedive and with the nerf to Soul Link so did survivability. As frightening as Warlocks used to be in PvP, the abundance of crowd control-breaking effects and immunities coupled with the sheer squishiness of cloth classes, resulted in the decline of Warlocks from the PvP scene.
In fact, in Season 5 so far, Warlocks are the least represented class, scraping the bottom of the ratings barrel. Lower even than the Arena-gimped Hunters. On the other hand, the one tree that's somewhat gimped in raids is the only way to survive more than five seconds in Arenas nowadays. Metamorphosis, as much as I hate that confused 51-point talent, gives Warlocks 30 seconds of nostalgic dominance before crashing back down to reality and exploding five seconds after.
3. The return of Affliction
The newly fire-centric Destruction tree is still very viable, with the hybrid Felguard / Emberstorm 0/41/30 spec popular with some raiders. This is tied in with number 1, but the coolest thing in late 2008 is the return of Affliction to the raiding scene. Sure, it's kind of crazy to play, with so many things to watch and having to constantly cast something, but it's cool that DoTs -- the hallmark of Warlocks -- now actually dish out some competitive PvE pain.
4. The rise of Death Knights
The Flavor-of-the-Month class seems more and more like a real alternative as more people seem to be playing their Death Knights than their old mains. Some of those people used to play Warlocks -- those players who wanted to play something sinister, apply a lot of DoTs, get a free mount, and have an evil pet. Why play a squishy Warlock when you can play a Death Knight in plate? As much as Metamorphosis tanking is fun in concept, being able to actually spec to tank is even better. Sure, Warlocks can summon friends, but who needs that when you can drag enemies to their doom?
5. The improvement of pets
If there's one thing I appreciate about the changes this year, it's the tailoring of demons to fit particular play styles. Demons aren't going to beat out Hunter pets in DPS anytime soon, but at least they're now much better than just fodder for Demonic Sacrifice. If anything, I love how it's all very thematic. Felhunters for Affliction, Imps for Destruction, and Felguards -- as well as the option to change it up -- for Demonology. I don't think demons are quite finished yet, but Blizzard has certainly taken great strides in the right direction.