Farcry 2
Operation Flashpoint: Dragon Rising
Fallout 3
Oblivion
CoD4: MW
Mass Effect 2
Woah dude, I hope that was a joke!
For me I would probably say Halo Wars. I looked forward to that game so much. I downloaded the Demo on Xbox Live when it came out, and thought "this demo must be like 1/4th of all that you can do in the full game!".. It was practically the full game, just with 2 of the "heroes" if you would call them that. And in general, the game is very spammy. <-- imo.
Other than that... Prototype! I thought it would be so much more! It was a decent game, but at the same time it wasn't really what I hoped for.