Freedom.
First, I don't /have to/ be in the office 9 to 5. I choose to arrive around 9am on most days, but they don't require me to. I also usually leave around 2pm so I can pick up my daughter from preschool.
Also, even though we have a good CTO, I have the freedom to choose some technologies, libraries, etc. to work with. We sometimes have a good argument about it, and sometimes it turns out I'm not the one who is right (and that's a good thing).
Freedom is much more important to me than anything else. In return, I'm flexible, too. If there is a tight deadline, I'm willing to be awake for all night to get things done (it already happened once or twice).