Game Material Physics
I was thinking about game physics while playing CS: Source (the physics bear absolutely no relation to gameplay, though, and I'm starting to suspect that will be nearly the case in HL - aside from 'tossing' random objects) and it seems to me that a much more accurate simulation of materials could be achieved by having three basic materials and interpolating the 'amount' any object had of each material.
You have, basically:
Crystaline (High-carbon steel, glass, CDs)
Plastic (Iron, ropes)
Elastic (Springs, erasers)
In short everything, when struck by a force would do some combination of 'fragmenting', 'bending' and 'bouncing' according to these properties. For example shooting a wooden door would have a crystaline effect (shatter a portion of the door), while shooting a filing cabnit would have a plastic effect (the location would be ruptured and dented inwards). Shooting something extremely tough, like a light steel door would maybe dent it but mostly just cause it to bounce around.
This is really just a combination of glass fragmentation and arbitrary terrain deformation effects combined with the half-life 2 collision physics, everything here already exists and I would think one could implement them all in a general engine and have it run on the higher end machines.
In some remote corner of the universe, poured out and glittering in innumerable solar systems, there once was a star on which clever animals invented knowledge. That was the highest and most mendacious minute of "world history"—yet only a minute. After nature had drawn a few breaths the star grew cold, and the clever animals had to die.