I like these. But I agree, the last one to "click" for me was Kirk, which is a shame with him being arguably the most important character.
I spent a while wondering why that was, because I think you've actually captured the physical traits pretty well. I suspect it has something to do with the magic of television, and how Kirk is presented as a character. If you take a look at some TOS scenes you will see that while the actor playing Kirk is by no means a big man, in most shots he is presented as one. He just looks big and buff on the screen. Most of the time he is portrayed almost as big as Spock, except in such scenes where Spocks height is being used for some particular effect (like when they meet.)
I would try to buff him up a bit more, and also try to lose the slouch... it's a nice character style, but it sort of goes against the spirit of the character. Additionally I think your Kirk has a bit too narrow face. Perhaps shave a pixel head height and use it to add buffness?
If all else fails, you could try to place the somewhat ambiguous characters nearby Spock and Uhura (who you captured very well by the way.) Sort of feed off their characteristic looks.
