Why you ask #5? Because id is taking blind stabs in the dark to try to make us think D3 is better than HL2 is some way.... People don't respond well to groundless (or unproveable) claims. Especially when they come off sounding as nasty as this one does.
How is it unproveable?
Havok, the physics engine being used for HL2 is easily accessible for anyone that owns 3dstudioMax (i.e. id software), and they even have a working demo of the physics without the 3d development program enviornment right on Havoks web-page.
Both cases, you can test the engine and see how much resource it takes.
Hell, even I know what Havoks capable of, and I didn't spend a dime on research.