effectively killed Stevenson's career, and the same could be said for Linville and his forgettable series. Burghoff didn't do much of anything after MASH (pun intended).
Hunter may have been an acceptable captain, but the chemistry of the Big 3 would have been much different. Playing Spock and McCoy against each other with Kirk in the middle just worked. This mainly due, in my opinion, to a quality that Shatner possesses and employs very well. It's the ever present and barely concealed smirk, and the "twinkle" in his eye.
Shatner has been much underrated as an actor since he became Jim Kirk.
Again my opinion, but having Hunter leave was a happy accident that resulted in the Trek we all know and love. If he had come back for the second pilot there may never have been any Trek at all. At least not like we know it.