I didn't realize just how hilly the terrain is until I played RWR on my 3D TV in stereoscopic mode using Tridef. The landscape is really important in this game. (It looked gorgeous in 3D BTW)

So, I think darkening the hill's shadows would help a lot. The hill shadows are very light, and darkening the shading to the landscape would allow players judge the topography more easily. Sometimes there's a small hill right in front of me and I can't even see it. Players could more easily visualize right when their character's head is going to come over that ridge so they can start shooting instead of getting killed.

(and yes I'm aware of the visual tweaking options and using isolines)

Actually they don't. They don't see the terrain at all. They sometimes try shooting through it even if it's "obvious" it makes no sense. Those cases would be flaws in map design or in need of additional helper structures to block view. Making the AI see the terrain would use up plenty of performance.

The day/night cycle and moving sun/moon also plays a role in this. There's a point in time when the shadows are very light (they fade away as we only have one shadow casting light source and the light source needs to flip to the other side) and for a brief moment the shadows are not seen at all.

"if there is an object between you and the enemy, he won't be able to detect you. " - wiki Manual

OK, gotcha. I guess hills don't count as an object. Anyway, darker shading would help. You guys should also trying seeing your own game in 3D, I think you'd be surprised. It looks real nice. I didn't really understand just how big the hills are until I did that.

Yup, hill doesn't count as an object in that sense. The cliffs do as they are basically walls rather than part of terrain.

Good point, should give 3D a try, my telly is 3D after all.