They are working on becoming World Champion Ping-Pong players.
Stupid is as stupid does.
I don't think Valve can simply rehash the book-like linearity and intense scripted events of the original without boring a lot of us jaded FPS gamers (Look at Unreal 2.) MOHAA already placed that concept on the torture bed and stretched it's limbs to the point where they've almost torn apart.
Assuming they can't truly innovate gameplay-wise however, I think they can still pull HL2 off if Valve successfully recreates the feeling of awe and bewilderment the large-scale set-piece battles in the original inspired in us.
I'm sure most of us can remember the first time we met those Marine bastards, the dam with the Apache, the cliffside battle, the rocket launchpad, the tramride, the headcrabs + elevator scene, the G-Man looking down on us, the first time we set foot on jumping-puzzle-land, etc. Diverse, imaginative, and most importantly memorable situations that are indelibly marked inside every gamer's head. Something that many FPS titles seem to lack nowadays.
"Nothing livens up a robotic hymn of doom more than an amazing pair of jugs." - Brak