Well for Halo fans like myself we have three new titles to look forward to this year, Halo ODST, Halo Chronicles, and Halo Wars, all of which (Chronicles excluded because I haven't heard enough about it just yet), seem promising.
From the game modifications department we have all of the DLC coming to Fallout 3, making it look like I'm going to be spending even more time playing it, something I hadn't thought possible. Then there is the Black Mesa Source mod that should finally be released this year, letting me finally experience the first Half Life in all of the current Source engine's glory.
In addition to all of that both Bioshock 2 and Mass Effect 2 should be coming out this year as well. Showing that it's going to be a damn good year for me.
Edit: As for the stuff I've already experienced, Mirror's Edge gets points for having an ending that put a smile on my face upon its completion, and Gears of War 2 managed to pull the series away from being a mindless shoot fest that the first one had it going towards by introducing some very likable characters, developing on the existing ones, and laying the ground work for an interesting story.