Co-op isn't what it used to be. Thankfully.

Mass Effect
The Longest Journey
Morrowind
These are by no means bad games, but they don't live up to the hype. Oddly, I initially found Oblivion to be disappointing in many ways, but then I became an Oblivion fiend. A game I liked that most people found disappointing is Star Trek: New Worlds.

I agree with Magic Online.
X-Com...I guess I'm the only one who likes all BUT Enforcer.
But Neverwinter Nights? Really? Wow...I didn't like NWN2 but NWN1 never leaves my HD. So many mods, adventures, etc....I also liked the main campaign and most of all, Hordes of Underdark expansion.

As a follow up to Baldur's Gate it just wasn't doing it for me. The move to 3D just killed it for me at the time and the game us butt ugly today.

The mod-ability of NWN is the game's only redeeming quality, as there are plenty of excellent mods.
But the single player OC ... ugh. Terrible.

Well let's see.
Fallout 3 definitely the most disappointing game ever for me. I still had some hope even though Bethesda's games are... don't appeal to me but in i got this crap. Oooh it's got Liam neeson and ... well 5 actors for the other 100 characters. No story. Combat where you can kill a big bag mgreen mutant with a minigun with lvl 2 char with a bbgun ( a little exaggeration but not by much) and V.A.T.S. ... UNISKIPABLE SLOWMO GODMODE and only the player has it. The environment while looking nice feels like the war finished maybe a year ago not 200, everything is still there, the water is still radited and no source of radiation in it. .... not mention what they did to special :/...
Ugh i hope Obsidian ent can create something better with this thing.
Half Life 2 - I never could understand why it was getting such high scores. It was fun playing with physics but it was a cut and paste job. So much was cut it wasn't even funny, and when you needed ammo you had unlimited supply nearby. And the boat ride.... far to long and tedious.
And DX:IW . Everything has been stated already.
that's pretty much it. Other games while disappointing were smoewhat more enjoyable as a whole or i wasn't expecting much in the first place.
Bioshock for an example. It might have been advertised at first almost "SS3" but once TakeTwo bough IG i knew it was only going downhill. So i left all my expectations down and therefore i managed to enjoy BS for what it was.
Ninja Gaiden Sigma and Sigma 2 as well. Well Just prettied up and broken and more or less PUSSY versions , wasn't so sad since i've still got the better originals.
KOTOR2 well with LA rushing the devs and cutting development time for months it couldn't have turned out any better anyway.
There are still some others i've forgotten right now ...

Might want to check out the Restoration Mod if you're interested in a playable version of KotOR 2.

I wasn't really disappointed much by HL2, but you are right on that part. I actually just found that was just a engine update to the old HL1 but they had to come with some plot. Not really well executed in the long run. T'was also a boring shooter if you ask me, HL1 was a much better one. But it had too many funny moments which covered it up.
"Pick up the can"

My list is :
1. Blood 2 - I remember how awesome the first game was , and this is the game that got me into Multi-player on the Internet and clan play . I remember playing the first Blood on the TEN network man I miss those days . Monolith really screwed up using the money they got to make this game and instead choose to focus on Shogo which turn out alright .
2. The new Wolfenstien - What the heck happened to the multi-player here ? Remember levels like tram , beach front, and castle level ? Wow what a let down for sure .
3. Tribes Vengeance - How do mess Tribes up ? I mean come on man .
There is a lot more games , but for me these were big let downs considering how good the first games usually were .