Unlike hunters, warlocks only have a limited choice of pets and it's fairly easy to determine the strategy against a pet and their master. In the case of a Felguard, it will even dictate what spec the warlock is, allowing you to figure out what to expect. Most warlocks will have the Felhunter out because it affords them the most versatility on the battlefield. Felhunters can Devour Magic, and considering the number of magical buffs and debuffs going around in a battle, this makes them ideal pets to have out. If you're a caster, beware of Spell Lock, which good warlocks will have off auto-cast.
The good news is that most warlocks will have it on auto-cast. It's too much to micromanage for casual, occasional PvPers otherwise. The bad news is that the spell is a 3-second silence, which means it'll have an effect whether or not you're actually casting anything. If you get caught casting something, you'll be locked out of your school for an even worse 6 seconds. If the Felhunter charges at you and Spell Locks you right off even if you're not casting anything, that should already indicate your opponent's skill level. They don't deal much damage, so the only reason to kill them is if they're being well managed. Keep in mind that they're no longer as easy to kill, either, so in most cases, going after the warlock herself is better proposition.
If you don't see the warlock with a pet, she's probably got a Succubus lurking around somewhere, so be wary of getting crowd controlled with Seduction. Just remember two important things -- one is that Seduction breaks on damage, so it's not a good idea to waste your trinket on it unless you or an ally you intend to help is really close to death. The second is that Seduction shares diminishing returns with fear effects, so allowing the spell to take effect works in your favor. Of course, keep in mind that most warlocks only use Seduction to keep one opponent out of play while demolishing another. The Succubus is also the easiest warlock pet to kill next to the Imp, which never sees PvP play, so going after it is never a bad idea.
The Felguard is a demonology warlock's signature pet and is arguably the strongest demon a warlock can bring into battle. It has the highest health, so targeting it would be a bad idea. If you're a warlock, you can simply Banish any demon, but other classes will need to be more creative with their crowd control to keep the pet out of play. The Felguard can stun, and most warlocks will use it as an opener. The spell has a minimum range of 8 yards, so getting into melee before the warlock can pull it off sometimes works, but it's usually best to just 'eat' the stun. It has a short, 3-second duration and a longish thirty second cooldown, so just absorb it. It should be noted that Felguard has a small mana pool and very mana intensive spells. If you have a means of siphoning mana, targeting the Felguard's mana pool should greatly hamper its efficiency in battle.
Sometimes, and because it's often the only place they get to do it, warlocks will unleash an Infernal (or even a Doomguard) in the Battlegrounds. Now those are fun. They cause a short stun when called upon, but mostly they just wreak uncontrolled havoc. Ignore them. They're too big to bother with, and they make warlocks perfect targets because Infernals can't be micromanaged and thus warlocks can't use them to save themselves. If you see an infernal on the battlefield, find the summoning warlock and zero in. They're practically pet-less for about a minute. CLARIFICATION: Infernals can be controlled, but they have no special abilities to speak of -- no crowd control abilites, no activated attacks, just one flaming hunk of mass.
Occasionally, warlocks will use a Voidwalker to PvP. This might seem funny at first, but be wary of the warlock who has it out. The Voidwalker has extremely high health and provides excellent buffer when combined with Soul Link. These pets can also be Sacrificed to give the warlock a shield buff that absorbs damage. In general, expect a warlock with a Voidwalker to be able to take on a lot of damage. Rogues should also watch out for Consume Shadows, which affords the Voidwalker some form of stealth detection.
That's all we'll discuss for today, but when we continue we'll take a look at the warlock herself and what you should expect of the three different specs and how to counter them. Fighting warlocks these days are just a tad more complex than the fear-fear-fear-dead encounters of old. They're certainly much easier to handle now, but also a whole lot more fun. Until then, stay on your toes and don't be afraid of battle. Unless, of course, you're Feared.