I loved CoD4, but I'm also a big fan of the old Red Faction games.
Red Faction in my eyes was famous for its physics and destruction of properties. If I remember correctly, in Red Faction 2 for the PS2 you could actually dig tunnels in walls with grenades and rocket launchers, and then make caves that lead to new areas.
I always thought that was so awesome.
I saw the physics demonstration months ago of Red Faction:Geurilla and I have to say it's freaking impressive. The buildings were so complex and each part was detachable from a single girder to an entire floor. When pieces fell off stress created weight and leaned the building a certain way, sometimes collapsing or just hanging there.
I can't wait to see that.
I'm pretty pumped for CoD:MW2, though. Can't wait to see what they do with it.