Well, I'm glad he's under contract for 3 movies. It sounds like he may be tired of playing the part already, which is too bad. I think his character is the least replaceable of them all, and I don't want to see Uhura with a different Spock, and surely I do not want to see Spock killed off. I hope that's not what "into darkness" is referencing...
This could be a number of things or nothing. Things that it could be:
Fandom: I think he took a lot of heat for playing new Spock as he was written.
I hope he knows that people were reacting to seeing a side of Spock that they weren't used to seeing far more than anyone actually thinking he didn't do well enough because he was fantastic in the role. Makes me think of drudging up my appreciation thread again.
Uhura: Okay, this is a passing thought, and I really hope all it is is a thought. He really doesn't' tend to play heterosexual characters with love interests. I think part of what might have drawn him to Spock is that you can see the character as asexual if you want to because he never really had any emotional ties to someone that way (that we could see). Then, imagine signing up for the role and reading the script only to find that your character is into kissing someone, and she's got boobies. Spock/Uhura are my favorite part of the reboot, but seeing as since Zachary came out a year ago and really is working for equal rights for that community, this could be a downer for him.
I really do hope there's no truth to this.
Or, this could just be contract renegotiation time, and if so, then I do support Zachary having the best deal possible without it being extreme. Last thing, he really could just be bored. We're only 2 movies in, the second one hasn't even been released yet, and already this franchise is having more than its fair share of problems.
So, that's my over-analyzing for the day.