Ok, ignore the regenerating trees, while that's cool, that's hardely any fun.
Now.
Imagine a world that actually does function without you. Imagine a completely hud-less immersive game world (map is a paper map, check ammo by looking in magazine, etc). Guns have to be cleaned, major wounds have to be tended to, etc, etc.
It's more like the original Far Cry (or Crysis), not sure if you played that, but you're given a level, and you've got a base to infiltrate, say, and you have a whole bunch of obvious possible ways, or you could play it completely randomly and go in from a crazy angle with some mental idea, and see what happens. Half Life this is not. 50km^2 of explorable savvanah, full weather + day/night cycles, this shit looks hot.