8. Physics
You have the opportunity to play through certain parts (on the PC version at least) and interact with speech bubbles along the way that reveal an almost directors commentary for key points in the games development. One particular developer said something that sticks out in my mind; he mentioned that if you notice something they have done or made, then they haven't done it correctly. They want the worlds they create to be natural and seamless, and in the case of Half-Life 2, it is. It is incredible how beautiful Half-Life 2 still is. The textures and animations may have aged in the last few years, but the feel of the world is timeless in gaming. The bobble of discarded paint cans in water, the rag doll movement of the combine during explosions or just the general way Gordon feels rooted to the ground he walks, the game tells a great story, but its the engine that inhabits your every move and the level around you that keep that story engrossing.