Counter Strike: Condition Zero
Well, where do I start .. CZ was supposed to implement bot functionality, and I half-expected the game to include new and better improvements both graphics-wise and even, gameplay-wise, instead, CZ was CS 1.6 shipped with Bots, and a cheesy single player campaign.
Counter Strike: Source
Same as with CZ, except this time, the source engine was able to give the game a whole new feeling, but not necessarily in a good way. Slow and chubby movements, a really weird aiming system, ragdoll over death animations, still crappy bots that for some reason seem to eat your system resources like a nasty virus. They made some great improvements to the game over time, but well, there are much better FPS now.
Mercenaries 2
Oh dear gods of Kobol, this one had to be the biggest letdown in video game history. How the hell do you ship such an awesome title without polishing it? The AI? Dumber than your average dork. Lack of a cover system, no fall damage, no damage from exploding vehicles, corny vehicle controls, and so much more.
HGL: Content Patch 2.0 never came out.
And a lot, a lot of others.