Valve, this is an awesome start. I know you are going to let the mod community take this and run with it, but keep in mind that the official version will also set the tone and be the core installed base. So don't just rely on mods for cool ideas: make sure you innovate the official DM too, adding the best ideas to the default game.
The biggest need:
-the weaponry in HL2 is just way too sparse and standard outside of the pulse rifle secondary and the grav gun. This engine cries out for creative weapons, or at least new twists on the old ones that made HL1DM so tactical and different (tripwire mines, hive hand, snarks, anti-camper through the wall guass gun, etc.) I know there wasn't time to add some of the cool weapons you were prototyping for HL2, but don't let the DM suffer for the lack of them: add em in whenever they are ready!
And something that would be cool to have be standard: make the crossbow bolts on a kill actually fling the body much farther than they now do. Let us pin people to the walls even if they have to fly several feet or more to get there. The body should fly along with the bolt until the bolt sticks it to the wall. Awesome. :)