Call of Duty is based on the Quake 3 (OpenGL) engine. In order to create new light effects, animations and A.I system, most of the old code had to be replaced. I wouldn't call it a beefed-up Quake 3 engine either since only a small portion of it was left intact. So what's the big thing that has been modified inside the engine? Probably the redesigned rendering system which can push 200,000 polygons per scene. The new TL also comes with support for pixel and vertex shaders which are utilized by the latest GPU's.