The physics in the single player part (judging from the movies from E3) look absolutely fantastic, but how will physics work in multiplayer? Won't there be a problem with sending all the physics information packets to all the players? Are you perhaps scaling the physics engine down for multiplayer?
Gabe Newell: Well, the simple answer is that there are client-side and server-side physics behaviors. You use client-side when maintaining cross-client coherence isn't important. This cuts down network traffice while maintaining the appearance of physical simulation throughout the world... it's definable per-object, so exactly what's client vs. server simulated is tunable."