its hard to describe exactly. part of it is the lack of persistance to things - for example, i drive through an enemy camp, and get attacked. fair enough. so i kill all the enemies, and make sure theres none left. i come back 2 minutes later having driven 50 yards down the road and turned round - and they've all respawned. also the fact that there's 2 opposing factions in the game - but i'm yet to see any difference between them (in that they both attack me, regardless of if i'm working for them or not), and i've never once seen them fighting each other.
oh, and it has the traditional problem of games like this... that soft cover has no effect on enemies shooting. they know exactly where you are, even if your the other side of a bush they cant possible see through - because their targetting engine only worries about *hard* cover, like walls and stuff.