How could a theoretical/quantum physicist be retarded? and i know your going to say thats because he is a "theoretical" physicist, but no he is not like Fantastic in Fallout: New Vegas, Gordon Freeman is far better than that scrotum muncher. The reason he doesn't have a voice is to not ruin the immersion factor of the game. Also, ValVe doesn't want to end up with another M situation like Nintendo has to deal with.
And yes, Chell may be mentally disabled, but she solves physics puzzles like a champ. then again, i'd like to see you awake from cyrosleep after god knows how many years and be fully cognitively aware.