I didn't think mgs4 disapointed at all when it first came out. It wasn't until a couple playthroughs later i started seeing the flaws and even then i still think it is a very solid game. The story, character endings and boss crew are the only disappointing elements off the top of my head.

Dark Souls 2 was disappointing for me, as much as i tried to like it. Ironically it doesn't really have soul.

HAWX 2 was probably the biggest of disappointing games, with Ace Combat: Assault Horizon not far behind.