Passion.
With the original Kirk you felt the character wanted to be a starship commander. There was nothing else he desired more and he pushed himself to become one of the greatest in history. No out side stimulus was needed. He had found his best destiny. His love for the federation drove him forward.
New Kirk, had no passion. Nothing drove him but circumstance. By the time he was talked into going to the academy you already had the feeling his life couldn't get any worse. Starfleet was just an option to get him out of his current miserable state. No love for the federation or what it stood for, just another job.