Thank you for not mentioning Killzone 2 in your OP, because from what I've heard, it's anything BUT generic.
You can't really blame the reviewers for GTA IV. They hadn't seen a proper Grand Theft Auto in four years, Rockstar could have released crap in a box and everybody would love it. People only started disliking it after they saw how unnecessarily realistic it was. That's why IGN hasn't given it Game of The Year. That's why GameSpot hasn't given it Game of The Year. They replayed it.
Clearly, most people who gave it the overall award didn't replay it.
People didn't like it because it WASN'T a rehash. People wanted a rehash. GTA went in a new direction, and people didn't like it. A direction where realism > fun. That was never what GTA was about. People wanted a generic game. They didn't get one.
Gears 2 was very similar to Gears, but that's no bad thing. God of War II was similar to God of War, and it was brilliant, even better than the original.
The only game which I think has changed greatly from its predecessor is Resistance 2.
EDIT: Let me elaborate. They did some things right, and some things wrong.
Right: Online co-op, improved controls, updated graphics, improved competitive online.
Wrong: Two weapons at a time, no co-op campaign, no vehicles.
Unchanged: Story makes little sense and is hard to follow, the base ideas for the weapons, the somewhat likeable main character who may or may not be in Resistance 3. <Resistance 2 spoilers
(Former) Lead Moderator and (Eternal) VGC Detective 
