Mater of Orion 3 - The first and second games were simple and fun, moo3 was a mess, almost every part of the game was broken or unfinished. The single thing it did better than the previous two titles was the ground combat.
Black & White 2 - The original was a massive let-down, but in my mind the entertaining creature AI, good looks, and nice ambient track saved it from being a total failure. B&W2 pretty well removed the creature from the game, was pathetically short and easy, as well as being repetitive and mind-numbingly dull. Graphical glitches caused severe issues on many NVidia 6 series cards (probably the most common graphics card at the time).
Hellgate London - Was just an awful game. It felt like an grind-fest MMORPG, except I was the only guy playing. LAN play did nothing to resolve the issue.
Spore - Was just way too simple. The programmer in me is very impressed with how the game works, the way it works seamlessly online, but the gamer in me cries. I would have been far more entertained with a massively fleshed-out cell stage game. The creature stage was fun for a bit, but tribe, city, and space were all just a drag after the initial newness wore off.