"didn't change his facial expression the entire episode", "far less dynamic"... How long ago did you guys watch the episode? Maybe you're basing it on the snippets of Cage scenes in the Menagerie. You've got to watch the whole Cage episode in its entirety. Yes, Pike is portrayed as a very serious captain who rarely smiles. But he does show a range of emotion. And... just look at Kirk in early S1 episodes compared to later on. Huge difference. Hunter didn't have a chance to develop his role.
Granted that one episode is not sufficient for an actor to "develop his role." But if you compare the two pilots you'll see that Kirk runs a whole range of emotions in WNMHGB whereas Hunter generally looks constipated throughout the Cage. His facial expressions change very little between the heart to heart with the doctor, discovering that he is in a cage, getting turned on by Vina, etc.... In short, Hunter doesn't really look like he wants to be there. Does anyone know if he thought the show he was a part of at that time was weird? I heard a Nimoy interview in which he said that one of the reasons why Hunter didn't stay on was because Paramount could not get him the movie deals he wanted.