Its all used in the combat enviroment - you really should watch some of the videos.
And thats the thing - the enviroment is so detailed. and interactive. for instance. in a run down part of town the main character is being hunted down by gaurds, he kicks open a door (no need for keys here - tho at last a rocket will take the door of its hinges) head inside, and pushs a table in front of the door. it will hold for a while not for ever.
but its the type of interaction that in the given situation you would prolly do yourself.
heading out the room and up some stairs he comes under fire through the windows, and a gaurd from the top of the stairs. caught in a cross fire most games would just have u shoot your way out.
Gordon has a number of options, using his grav gun he tears a radiator off the wall and uses it like a shield to deflect the gaurds bullets from the top of the stairs. and then hurls the radiator back into the gaurd. there is more rubbish and stuff at the top of the stairs, he kicks this down the stairs to slow the persuing gaurds, again it wont hold but maybe he can get away. it saves ammo too.
none of this was needed, perhaps some well placed shots in the gaurds head would have been just as good, but at last I have real life options due to a good physics engine.
One of my favs is watching a gaurd lean over a railing - gordon is having a shoot out, a stray shot clips the railling and breaks the bars, the gaurd falls to his death. - its not really that scripted either. there hundreds of raillings but nearly everything on show can be interacted with.
Another nice touch is when your in lab with some proffesor, hes telling you about some part of the plot, your free to walk about and do what you want, HL doesnt have cut scenes, everything is always from your eyes. by accident you bump into a computer, and knock it on the floor - he stops mid sentance to tell you to be carefull in a somewhat irate manner.
Actually something I do like about this game more then anything it seems there is always a choice as in how to deal with a given situation. most games normally result in the only answer to be using a gun. This game gives me the option to use my brain to trap or slow down my enemy.
In a violent age I think this is a good thing.
anyhows please go watch some of the videos, which are nice because they arnt videos captures but actually use your computer to render the scene. pretty nifty.
http://www.planethalflife.com/half-life2/videos/
gah i sound like a salesman - I just dont want ppl to miss out on somthing which i feel is a real moment in gamming.