Castlevania: Lords of Shadow. The God of War, Dante's Inferno and Devil May Cry type gameplay was so boring (to me) at this point. So when I played this game, I was very underwhelmed and disappointed. It's not a bad game, it's just not what Castlevania should be. Castlevania is known for being a side scrolling savant, it's just the way she goes. The transition from side scrolling to high angle Kratos chains massacre was just not my cup of holy water.
I have no problem with change, as long as it's a change for the better. If it ain't broke, don't fix it.
Also, I realize this isn't exactly a sequel by definition, it's more of game within an on going series... but I don't care.
I'll put in Assassins Creed: Brotherhood in as a legit sequel that disappointed me. I played it for an hour and was very bored. Can't really pin point why either, ACII is my 8th favorite game ever, they aren't much different.