Basically the only real difference was that
he liked to whisper a lot. I think Disco's Mirror Universe episodes knocked ENT's completely out of the water.
Somewhere else in the thread, Post #13, I said that Scotty got his analysis wrong. Cutting and pasting from the other post...
I was going to approach this from one way but as I started typing out my argument, it didn't work, so I'm not going to. Scotty said that the transporter created two opposites. So if Normal Kirk was at 50, then one Kirk was at 0 and the other Kirk was at 100. "Evil" Kirk could have his own moral code, it's just the exact opposite of "Good" Kirk's. That's the angle I was going to come from but we see
no evidence of a code in "Evil" Kirk at all.
So it probably really
is a matter of Kirk being broken down into superego and id. Scotty's an engineer, not a psychiatrist (like Kirk said later on in
Generations 
). He was making the best guess he could. Spock and McCoy have it right later on. McCoy says we all have our darker side. And then Spock points out how he as a whole person wins out of over his Vulcan half and Human half. Spock is a walking, talking Ego controlling his two halves. So there we are.