Needless to say, this is still every warlock's bread and butter. Fear has been nerfed to the point where it's no longer quite as, well, fearsome because it has a much lower damage threshold and shorter PvP duration than the old days. Still, it's something every player should expect from a warlock. Because Fear is absolutely terrible in a PvE environment, warlocks relish using it in PvP. In the Battlegrounds and world PvP, they'll tend to use it less strategically and with abandon. Remember a couple of things: it suffers from diminishing returns, so even if warlocks can recast it, it won't last as long. Is it worth a cc-break? For classes who don't have built-in counters to fear effects, yes. It also has a 1.5 second cast time, making it perfectly interruptible and locks the warlock out of the Shadow family of spells, preventing most of their DoTs and other crowd control effects. It's arguably the most interrupt-worthy spell because it occurs before losing control of your character, and because their other spells are instant cast.
Speaking of instant cast spells, Death Coil is a spell that warlocks use in a pinch to peel. Some warlocks even use it as a precursor to Fear. The notable thing is that it is a horror effect, so warlocks can actually stack Fear on top of it. It's almost never worth it to trinket out of a Death Coil, so don't be baited into it. Be aware of the difference and never mistake being Death Coiled for being Feared. Instead, wait for the inevitable Fear. It's an instant cast spell, so there's not much you can do about it. Just be mindful of its two minute cooldown.
Corruption and Curse of Agony
Two of the warlock's basic Damage-over-Time spells, Corruption and Curse of Agony are instant cast, so they're uninterruptible. They also don't share the same debuff slot, so most of the time these two work in tandem to bring down opponents' health. The good news is that there are a lot of utility curses that take up the curse slot, so it won't always be a damaging spell. The bad news is that only druids, mages, and talented shamans can remove curses. Still, if you are able to shake off a curse or magical effect (Corruption), you shouldn't allow them to stack. DoTs soften you up for the kill, so keep your health up with defensive dispels and self-heals if you can manage it.
Recently at the office (or what passes for the WoW.com office these days), one of our writers posted a pic of a Demonic Circle on the ground and asked, "what is that?" and noted that she'd never seen it before. Interestingly enough, when warlocks started getting the spell at Level 80, Dalaran was filled with them. Now that the novelty has worn off, you're more likely to see them in the Battlegrounds. Good warlocks will always have a Demonic Circle prepared. If they don't, take confidence in the fact that you're probably fighting an average 'lock. Always keep your eye on where the Demonic Circle is and always fight near it, or at least know what direction you should head when they get the spell off. The teleport itself has a fairly long range, so if they manage to get away from you using it, they can create a large distance. Just remember two things: it doesn't allow them to escape any crowd control except snares, and it has a long 30 second cooldown. At best, force them to blow it and take away one of their few escape mechanics.
Howl of Terror
This spell is one of the warlock's other means of escape. Howl of Terror is an awesome peeling spell but it suffers from two big drawbacks -- it has a casting time and has a long 40-second cooldown. Affliction warlocks can reach deeper into the tree and make it instant, so keep an eye out for their spec. The spell shares diminishing cooldowns with Fear, so this is also worth the trinket use. Because this is a peel for melee at 10 yard range, if you're a ranged class, you needn't worry about this so much.
Seed of Corruption
In many cases, warlocks will opt to place a Seed of Corruption on a target instead of the run-of-the-mill Corruption. In the Battlegrounds, where it can get really chaotic, it's an awesome spell to put on a focused target. Unmolested, warlocks can have their fun placing a chain of Seeds of Corruption on enemies while hiding a fair distance from the crowd. Pay attention to this spell. They won't cast it when they're facing off against you solo, but they will use it with abandon when against a group, so beware of those instances. It is a magic effect.