If you have Steam, download all the videos for the game and see for yourself.
Unbelievable enemy A.I, awesome graphics, almost everything around you can be manipulated, awesome scripting, and of course, hopefully awesome storyline and gameplay like it was in Half Life. I already know the face animations are incredible, and the voice acting is top notch.
But the engine. Thats where the revolution will come.
You like whats been done with the Half Life engine? Counterstrike, Day of Defeat, Team Fortress, Natural Selection, etc. Wait till you see what will be done with HL2.
Not only is the engine vastly superior and versatile, it will be easier to create mods, levels and animations.