"Congrats, now get out of here" endings... That must be the most frustrating of all. When you spend weeks/months/years trying to get to the end of a game and you finally do, I expect a big reward, not a pat on the back...
I'll make an exception for Wonderboy, as that "congrats" ending is epic. "Yep, the dragon was really a robot yada yada". That just made my jaw drop in amazement.