I think Daikatana perfectly defines "let down".Oooh, another one for me would be daikatana - oh dear.

Nevewinter Nights II.
Gorgeous character & environment graphics, but... hideous inventory graphics, clunky modelling, irritating cut scenes, aggravating character voices, nonsensical party system, and a criminally outdated "area-based" environment (unlike DS II or WoW, which has a true 3D environment).
Having played and loved all three of the original NWN series, I was hopelessly let down by NWN II. They had the opportunity to fix all the silly things from NWN, but chose to keep them in and add a few more.
Valve.
Its as simple as that. Thats the only developer that keeps me believing in PC gaming.
Star Trek: Elite Force 2
I really enjoyed the first game but the second was just crap.
ever been so dissapointed with a game that it has put them off gaming for good?
Trying to play in co-op was pretty much impossible because it was so badly thought out and implemented. The camera was enough to drive you to suicide. The game-world was terrible and felt like a bunch of totally detached levels stuck together. The graphics were uninspiring and ran like poop. Bad, bad, bad 
Locomotion!
How Transport Tycoon (over 10 years old) has still not been improved upon i will never know!