Gamasutra has an excellent article about the Phyical Gameplay aspect in HL2.



The Half-Life saga has been praised worldwide for its innovations in gameplay mechanics, from storytelling and physics to revitalizing the FPS genre. So, there was great expectation over Jay Stelly's talk on physical gameplay design in Half-Life 2 last Thursday at the Game Developers Conference. It was a session that examined Valve's approach to game mechanics design, and how physical interaction methods were devised for last year's blockbuster.
Read more about it here