[rkon]

Better yet, why hasn't anyone created an app that just simulates a guest mode? A launcher with two sets of home screens would be perfect. Or, you just disable your preferred launcher when handing your phone to a friend, revealing the mostly barren stock screens. It's as simple as hiding icons, since the apps might as well not exist if the icons aren't there (access doesn't actually need to be explicitly forbidden, per se).

Maybe I'm missing something important here, but it seems many apps on the Android market are just a few tweaks away from doing this already?

[pyre]

Not all games have a separation of user data. Some games basically only have one save slot, so that starting a new game wipes out the save data.

[freehunter]

This caused me to be beat upside the head with a Gameboy repeatedly when I was young when I hit "new game" on my cousin's Pokemon Red.

[HackR]

If you were my cousin who did that to me, you wouldn't exist today, to put it lightly.

[nopassrecover]

This was done to me by cousins and brothers maybe 4-5 times (with very well developed games before each time). I understand the frustration (although what you describe sounds a bit full on) particularly as I'm pretty sure you had to start a new game then save.