Not to be overhyping the game TOO much..but honestly it's amazing what Valve has done with HL2's engine.
The physics engine in the game is fantastic. The video shows it all. You can basically do anything you want. If you want to throw a barrel against the wall, have it bounce and knock over a nearby wooden ledge carrying a bunch of bad guys on top, it's possible. There's vehicles to drive around in. There's huge 60 foot spider enemies that knock over buildings and impale ppl. The amazing thing about this is that these events are NOT scripted into the engine. They all take place real time and are all rendered on the fly. Honestly, words can't do justice to everything I saw in the video. Here's like link to a reviewer that was at the demo at E3:
http://www.gamespy.com/previews/may03/halflife2pc/
It sounds like a lot of hype, but from what I've seen, this game will surpass Doom 3 in many ways. Doom may be the scarier game. But the Source engine for Half Life looks more impressive so far.