At last year's Blizzcon, the WoW class discussion panel stated that Warlocks are the "best dueling class" in the game. Having enthusiastically played a Warlock for over 2 years, I can honestly say that there were very few duels that I should have lost. The ones I did lose were those where I either messed up (which happened plenty of times, especially against certain Rogues I know...), or didn't take all too seriously -- a side-effect of playing a very dominant 1v1 class. But when I did take things seriously, to say that those duels were a walk in the park would be a gross understatement.
I know that sounds kinda egotistical, but it totally isn't. It has just been a part of the class is all. And not much has changed in Patch 2.4.
Warlocks in general have an uncanny ability to rock opposing players to their core. The pressure of facing a class that can do immense damage over time is indeed overwhelming to some. DoT, run, burn. This can cause hesitation, and at the same time, a debilitating, hurried effect that can easily press the enemy into errors of approach. The fact that Warlocks are built in such a way makes them very difficult to overcome, and watching a Warlock that has mastered the art of exploiting his abilities is definitely something to appreciate. Kinda like a chef in a kitchen of terrible ingredients, and who can kick your butt all over the place without ruining the soufflé he's got cooking in the oven.
To me, that's what Warlock PvP is all about. It's about controlling the battle via finesse and forcing that 1v1 scenario we dominate so readily. Being able to do so lets everyone watching know that you're in control. And, that if needs be, they're next.
Changes are incoming, however, as Blizzard has already stated that the Warlock-Mage relationship will be undergoing a transformation in the WotLK, which will make Mages much more competitive against the Warlock class. Other classes have already seen buffs based upon Warlock dominance, and many more changes are probably still to come. But the fact remains, even with all of these changes, that 'Locks have quite the trickery up their sleeves and are indeed the best 1v1 class.
With the proper gear and class-knowledge, we will always be extremely competitive, as long as we take ourselves and the things we do seriously.