I was absolutely blown away by this game's demo when it came out. The physics engine is just amazing.
An example of its greatness: I had a sniper get an interrupt on a German soldier who was clambering out of a window that was behind a bit of a corner. I fired, but the shot "missed" the exposed part of the soldier, instead entering the brick wall of the corner, passing through it to hit the soldier, and finally passing through him to knock out another hole in the wall past him. There are no canned death animations in this game, and the soldier was spun around in a half circle to end up sprawled half in and half out of the window.
Ever since I saw that in my first mission, I've been hooked. A game engine as powerful as this having been made with the full intention of being completely moddable? This is definitely one game that's a must-have.