IwubCheeze: My picks for disappointing sequels:
Blood 2 - Not much needs to be said here
Descent 3 - The wide open areas but slow moving weapon projectiles made playing this game a chore
Deus Ex: IW - Mentioned a lot in this thread already and for good reason
Diablo 2 - On release, I didn't like it, but subsequent patches improved the game a lot
Farcry 2 - They did so much right and so much wrong at the same time, it had the potential to be so much better and it kills me to put the game on this list
Gothic 3 - Still enjoyed this game a lot but some things were still off
Half Life 2 - Just over hyped I think
KKND 2 - I never finished this game, the bugs killed it for me. Loved the original though
M.A.X 2 - Absolute garbage compared to M.A.X 1
Medal of Honor: Pacific Assault Pure trash. never finished it and sold it off
Monkey Island, Escape from - Still enjoyed this one even though it lacked the charm of the previous game
Painkiller: Overdose - There was something missing here the original Painkiller had
S.T.A.L.K.E.R: Clear Sky - The broken faction war system annoyed me. Gonna give this game another try
Starcraft 2: WOL - Pure disappointment and no desire to play it again, never bothered with anything after WOL
The Elder Scrolls 3: Morrowind - To me this game was a huge step down from Daggerfall, I'm gonna to give this game a second chance as it looks like mods will fix some of the gripes I have with the vanilla game
Turok: Evolution - Just a meh game, didn't know pterodactyls exploded if they turned to sharply either
Unreal 2 - Still enjoyed this for what it was though
Looks like I'll be adding Commandos 3 to this list when I get round to it
Yes to almost everything. Except Diablo 2 (although you limit your disappointment to the release version) and KKND 2. Both were great LAN games.
Half Life 2 is one of my biggest disappointments. We played huge amounts of Half Life via Sven Coop. I've avoided the second part for a long time because of the stupid Steam client and when I finally got it in the bargain basement everything was wrong. It sounds, looks and feels wrong.
Unreal 2 was OK.