I agree with UrbanRat's overall point, but I think you're spot on in your last sentence about Just Cause 2. Game world was huge and beautiful, but I just felt like it was so cookie cutter boring. No area felt unique. Seemed like the same cars, the same traffic, the same people walking around doing the same things.
Unique areas are really what make something memorable. San Andreas was huge, but had these very distinct feelings depending on the area, and what made the wilderness feel so isolated (like when CJ gets dropped off in the middle of nowhere after the Los Santos chapter), was that you had Los Santos as this over-bearing inescapable city -- getting shot at at every turn, congested, busy.