Reasons I hang onto certain games:
1. Single player campaign is so good I might want to replay it again later (Portal 2, Uncharted).
2. Extra content that might be interesting (Dragon age origins, LBP)
3. Splitscreen/same screen well implemented local multiplayer. (LBP, Portal 2, Motorstorm)
4. My kids might want to play it when old enough (LBP, Lego games, Ratchet and Clank)
Online multiplayer doesn't really do it for me. I only play GT5 online and I would have kept that game anyway. I'm actually more inclined to trade in online multiplayer games early, better value and usually more forgettable single player campaign.
Best thing developers can do is make a great single player campaign that's fun to replay. Don't make it drag on, but give it the total package, great graphics, music, and gameplay. Or great local multiplayer and I'll never trade it in.
