Infamous 2. The fact that there are split endings if you are good or evil, and there are different electric powers if you are good or evil. You finish the evil story and you think "huh, Now I have to do the good story" Then you finish the good story and you want to do the evil again.
I honestly didn't feel that way with inFAMOUS 2. For me it was more like "Okay, I finished the good ending...now I have to play through the whole game again just to get a different trophy." Because really, not much of the story actually changes in inFAMOUS/inFAMOUS 2 when you choose the alternate.
This is actually something I'm finding with a lot of sequels. That they have less replayability than the first in the series. (There are some counters to this).
Dead Space, for example. I played through Dead Space 6 times (probably 7 total). I played through Dead Space 2 once. And didn't have the urge to go through it again the way I did Dead Space (1).
The same can be said for inFAMOUS. I played through inFAMOUS about 3 times, and I only played through inFAMOUS 2 once. I don't know what it was about the games, but for some reason I found the first games to be more enjoyable or more enjoyable to play through again than the sequels.
On the other hand, Uncharted and Uncharted 2 is the opposite. I took a long time before I came back and finished up with Uncharted: Drake's Fortune (getting all the trohpies and finishing the game on the hardest difficulty). But I played through Uncharted 2's campaign, I think, 2 times (probably 3 total). And played the hell out of the multiplayer.
Anyway, I dunno, stuff and things, as always.