Well, I guess "big" is in the eye of the beholder, but for the fairly easy task of starting a Twitter account (if you don't already have one) and following @Warcraft, you could win a myriad of WoW TCG loot card codes. Ever wanted an Ethereal Plunderer, a Spectral Tiger, a Magic Rooster, or any of the other super-rare vanity items? Now would be a good time to take part in that whole social networking thing. You could even follow the official WoW.com Twitter account too if you're so inclined.
Blizzard has already selected a ton of winners, and the contest runs until December 11th, 2009, so you've still get plenty of time to win. Official rules are here.
It looks like Blizzard learned something from their last contest, since the prizes seem to be handed out immediately upon them choosing a winner. Of course, I suppose it's easier to get digital codes to people than t-shirts, but I also don't think it takes two months to mail a t-shirt. Either way, if you win this one, you get pretty much instant gratification. The best kind of gratification.