Just Cause 2 is definitely a short increment game, I liked that I could jump on for 30 minutes, putz around, then turn it off. I actually felt rockstar lost that a bit with RDR and GTA4. They were more story intensive, and that meant more time intensive. I get what you are saying though, I just like to have a game like that around for a quick spin.
Saint's Row 2 does that even better, for my money, and it has a fairly good story.