For me it's the level of detail and complexity. I would be hard pressed to think of any other platoon-battalion level strategy game for the late WWII era that captures the immense variety of vehicles, weapons, formations and battles that CM does. You don't just control Panthers, you don't just control Panther Gs, you control early, mid or late model Panther Gs.
One thing I love about the manuals is the encyclopedic guide to all the weapons and units at the end. I often just read that part of the manual for fun.