Those lucky gits at GameSpy have managed to get an interview with Valve's Doug Lombardi about Half-Life 2, which by now needs no introduction:
GameSpy: Objects in the game are much more detailed, featuring a lot more than just static crates. How does all this play into the game?
Doug Lombardi: What we've done is move from textures to materials when we get down to how we build things. In the old world, we would take a 2D texture, a piece of art, apply it to some polygons, and then it would just sit there in the world unless we wanted a very specific functionality on it. A set parameter would allow you to slide a crate (or not slide a crate).
In Half-Life 2, we've moved from textures to materials so everything that used to be a texture, whether it is wood or metal or whatever, has properties assigned to it for weight, consistency, etc. that all physical objects would have. So what you have is that a bottle or a crate or a barrel sort of take on all of these properties and allows the player, and non-player characters, to manipulate these objects. Things that you thought were safe to hide under or behind are no longer so because they can be blown apart, moved, or what have you.
All objects in the world can be used for alternative gameplay solutions as well. It also keeps players on their toes because they aren't safe in places that they thought they would be.