One game that comes to mind is saints row 3. It was so watered down compared to saints row 2

Unreal 2

Not a bad fps by any means (a solid 75%), but nowhere near the excellence of the first game. Some major gripes include: overabundance of enemies, deeply dissatisfying endgame and weird roleplay elements. Seems like the game could've been a whole lot more. I wouldn't be surprised to find that development was rushed.

All of those are fine choices.

Invisible War was particularly disappointing given how good Deus Ex was/is.
I don't know how they managed to make a game where the design was inferior in every way to it's predecessor. Ugh; those horribly small maps as well. All they had to do was clone the first game to make lots of $$$$

The weird 2D/3D mix in Commandos 3 didn't float my boat; Commandos 2 still looks gorgeous. Didn't like the timed mission aspect either.

I can't say I was particularly shocked by Bethesda's take on "Fallout" given how Oblivion turned out.

I could probably add Curse of Monkey Island to the list although Escape from Monkey Island is meant to be even worse. I don't consider those to be real sequels without Ron Gilbert at the helm.

XCOM APOC is a horribly balanced mess of turn based + real time combat with terrible battlescape character models and ufopedia art. XCOM vs Barney the dinosaur + headcrabs.
There's actually some good stuff buried underneath all the garbage - mainly the cityscape.

I wasn't that impressed by Thief 3 (Deadly Shadows) either - I did like Robbing the Cradle, that left an impression.

I hated Metal Gear Solid 2 - bought the clunky PC release and tried playing using keyboard controls!
Nothing like the first Metal Gear Solid (i.e. PSX) game at all.
I wanted to be Snake not Raiden - didn't read a review; just assumed it would be directly following on from the first.
All the monotonous dialog and wacky story/characters. Trolling? Or meant to be an insult?
Splinter Cell did just what I was looking for fortunately.

Wow.

I too much prefer the original Witcher, but did you find the second to be such a disappointment because you found the first to be so great or did you find a lot of things with it you didn't like?

As for my list:

Diablo 3
Prince of Persia: The Warrior Within
Final Fantasy VII, VIII, X
Burnout Paradise
LEGO Star Wars Clone wars
Borderlands 2 before Gaige was introduced.

Oblivion easily takes that dubious honor. Remove everything I liked about Morrowind, and you get Oblivion.

Oh, man I'd forgotten about this title. Not only is it a horrible sequel its a horrible end to an iconic RPG series.

I'm going to name Deus Ex: Invisible War as another bust. The sad thing about it is that if they dropped the Deus Ex name it would have had a far better reception.

A game I haven't seen name yet is Unreal Tournament 3. The graphics were such an over valued and poorly implemented part of that game that it was released it a horrid state. Once it was patched it ended up being more of the same but much less of it.

It seems like I'm not the only one who was disappointed with Monkey Island 3. I don't think it's a bad game by any means, but it just wasn't the same without Ron Gilbert and I totally hate the art direction they took with the characters designs, that goofy looking guy is just not the Guybrush I knew.

But my biggest disappointment with a sequel was Kyrandia 3, they messed up the story from the first game, some puzzles and areas are incredible annoying and those ugly pre-rendered 3D graphics from those days are the worst, the art from the first two games was gorgeous. Kyrandia 3 could have been so much better. :(

Ditto and Ditto.

Loved TS.

My vote goes to Driver 3 (PS2). I loved Driver and Driver 2, I bought this game blind based on that... letting Tanner get out of the car was a HUGE mistake, it was very poorly done and made the game feel like a cheap GTA3 knockoff and almost unplayable. I struggled through it, but I learned my lesson at that time to NEVER buy a game blindly on release. So it wasn't a total waste.

~

Funny, the save system was my favourite feature of that game!
I only played this series recently, but I definitely agree that 3 was much worse than the others. I enjoyed Kyrandia 1 and especially Kyrandia 2, but I couldn't finish Kyrandia 3.

Now you're trying to pick a fight!

...anyways... Came to add the disappointment that is Blood 2.

Resident Evil 6... Game's alright in co-op if you're dicking around with friends. But as a horror game it fails even worse than RE5 did.

Yes, Monkey Island 3 was a let-down after the two brilliant predecessors. But with contenders like Ultima 9 I wouldn't call MI3 the 'Most Disappointing Seqel Ever'.

Diablo 3,Unrel 2,Fallout 3,Quake 4,all Alien games after AvP 2 and so on...wow thats a lot of dissapointment

Blood II

Diablo II
TES Oblivion
Deus Ex: Invisible War
Fallout 3