what can this new engine offer?
One of my favorite things in NS was the ability the marines had to change the environment through weldable stuff. Unfortunately with the HL engine this was very limited and had to be set up specifically by the mapper. I think with the HL2 engine there's huge, huge potential to make this an awesome feature.
Let's say instead of making specific points weldable, the mapper just litters the map with a few physics objects that can be used for welding (chunks of sheet metal or what have you). This is pretty much already built into the engine with physboxes, so no problem there. Now let the marines pick them up (tweak the code for the grav-gun), push it against a wall, and weld it there. as long as a point of the sheet metal is touching a wall (or other architecture) it will stick. Maybe you can't weld onto dynamic infestation.
Now let's say this welded chunk has a sort of strength to it, not an amount of hit points, but a damage threshold. So a skulk can't do enough damage to it to budge it. An onos, on the other hand, can bash in no problem.
This way if the marines close up a vent, it can keep the skulks away (and skulks will probably be the only aliens who can reach it for a while). If the marines want to block off a larger area, a narrow hallway or a hatch, they can do so knowing it will only hold until a larger alien comes along and bashes it in.
This seems a very logical and extremely gameplay-interesting direction to take weldables in the HL2 environment.