From Wikipedia: "The principal reason for the fork was to create a more community-based web application framework as many users were frustrated with CodeIgniter's lack of bug fixes and inclusion of new features requested by the community."
In practice, this means two things to me:
1. Whenever I'm in doubt, I can easily go to the code. I don't live or die based on documentation. Everything from the front controller to the templating library is accessible to any skilled developer.
2. It doesn't have to be a ginormous "all things to all people" framework like Zend or Cake or Symfony. (Those are great frameworks, I'm not knocking them). It can stay smaller because if there's any functionality I need that isn't there I can write it in without much work. And this is very idiomatic in Kohana. The framework encourages you to extend and overload.
The community forums are active and friendly, the only lack I saw (some time ago) was the documentation, but the code is very clean and well documented with concise and clean comments.
I used it right before my complete switch to RoR, I was happy with it, but php is still the big poorly designed language-mess which has always been.