Devil May Cry 2: Just.. meh, and if you're making a sequel to a game by Hideki Kamiya then "meh" should be the very goddamn last thing people think.
Modern Warfare 2: MW1 actually had a pretty good sp campaign. This one didn't.
Dragon Age: Awakening: While it's an expansion, I think it suffices. They just dumbed everything down. In a way it kind of seems like Dragon Age: Origins is the vastly improved sequel to Awakening, with Awakening being kind of a prototype that has some good ideas but doesn't really work.
F.E.A.R.2: Another example of "the first one was okay, let's do the same thing again only worse".
Fallout 3: Oh sure, New Vegas was buggy, but that's really the only serious criticism of it. FO3, however, was just dull. I guess the difference is that Obsidian can actually write.