Tough one, to be honest I am disappointed a lot more often with games than impressed by them these days. I find most of them far too short, completed in a few days, and largely just flashy graphics rather than any substance. Just so much crap released that people keep on buying which in turn keeps making devs churn out more crap.
Looking back at the year, I would have to say the biggest disappointment for me was Spore.
It really could and should have been something truly evolutionary (pardon the pun) in gaming. In the end though, it was just delays, promises and hype resulting in a limited, restrictive game lacking depth, intelligence and scope.