Number 1: Patch 3.2
In the end, it probably wasn't the best patch of 2009 -- lots of people wondered why we were fighting each other when Arthas was right there, and while lots of players ran Trial of the Crusader, it probably won't win any popularity contests against Ulduar or Icecrown Citadel. But nevertheless, it was a huge update to the game, with lots and lots of player interest, and coming as it did right in the middle of August, it was the biggest story of the year for us.
What will 2010 hold? Cataclysm is expected, and it'll definitely shake up the World of Warcraft. If the hints we've seen are any indication, Blizzard may hold another convention, and it might not even be in Anaheim. And as always, we'll definitely see some surprises. As the new class changes for the next expansion unfold, and new features are unveiled for the game (and players find their way to the end of Arthas' story), we can be sure to expect one thing in 2010: the unexpected.