Halos Spinoffs, Sonic, Doom, Duke Nukem, Lost Planet, Fable, Iron Man, Two Worlds, L4D.
Now I don't by all means some the things I mentioned didn't deserve sequels, but they could be been better.
L4D2 - They added new zombies, new settings, new scope guns, and melee weapons. All these could have been added onto the original without issues, so why the stand-alone idea?
Iron Man 2 - Very bad game, period, they could have atleast improved it GREATLY! Learn from the first one guys.
Lost Planet 2 - The first one was OK, but it lacked any characters which you could care about and the story was generally daft. The second one comes out, and forget the SP aspect of the game...plus the bugs are insane.
You see the point, sequels could have been better, as they stand now, they're somewhat average and worse then the first titles for most of them.