Mafia 2's details with GTA IV's scope.
I appreciate that a lot of the depth to M2's details could only be done due to the linear nature of the storyline, but how about having some random events in game that occur only rarely?
Lets take an example:
Lets say you have a scripted car crash, set to go off if you drive down a certain road at a certain time. A Taxi gets rear ended by a black saloon, drivers get out and the saloon driver shoots the cabby. Now most of us would have driven strait past when it occurred, but lets say you stopped and ran to help the cabby - gives you a reward or achievement. Or you chased after and killed the saloon's driver - a different reward or achievement.
You could drive that road hundreds of times and never see it, since it will only happen at a certain time of day and with a random chance of occurring even then. Which would avoid the "OH LOOK! Its a person you can meet! Look at them, no you must we make the camera point at them! See, we even turn up there voice volume so you see them! DO THEIR MISSION *****!" we got in GTA IV.
Plus since it could only happen once on each playthough you could only get one achievement at a time - keeps the achievement nuts happy and gives you a choice of in game rewards to go for. And of course you can just choose not to bother with any of them and get the "Unobservant" award for skipping too many of them.
Oh, and it would be nice to see it back in the UK again boys.