King Daniel wrote:
Kirk has explicitly changed from The Original Series - his father died, who we're told was his inspiration for joining Starfleet the first time around. The rest I put down to differences in actor interpretation and youth.
Another Kirk observation. He also seems to be a bigger klutz. Banging heads on rafters etc. However, Kirk prime could have been a klutz as well I suppose in his youth.
To be fair, he was rather hungover. We've never seen Kirk Prime with a big hangover. As for those who say Spock is very different, I'm not so sure. How do we know Spock Prime did not react in much the same way as Spock regarding his appointment to the Vulcan Science Academy "despite your disadvantage"? We know Spock chose an unusual path--Starfleet--in each timeline. There is nothing to suggest he was any less "pissed off" in the original timeline. As for his subsequent "emotional" behaviour, well, there have been enough posts that reference things like "THE WOMEN", his smiling, his past with the woman from This Side of Paradise, the flirtations with Uhura in the early episodes, his smirking in early episodes, and others I'm surely forgetting, to point to a less than fully stoic Vulcan control over his feelings in the TOS era. That makes his even lower degree of stoicism understandable in ST09 as he is younger--and it is exacerbated by the experience of watching his mother die (something he was spared in the original timeline until much later).
In the end, I think Kirk is the most obviously changed character when we first meet him. Spock's experiences in the main body of the story set him up to be significantly changed going forward. McCoy seems pretty much the same, only younger. Scotty seems to be a more frivolous character, though youth comes into play there as well. The rest have very little to compare too, so are free to develop in various ways. All in all, apart from Kirk (and even there, there are many similarities with Kirk Prime), it's actually rather remarkable how they all resemble the prime characters, rather than deviate in nearly unrecognizable ways from them. Part of why I found the film so relatable to my memories and expectations of TOS-era Trek.