The big new addition is tags, though I will need to update my themes to view those. I’ve never been quite comfortable with the Categories system in WordPress, perhaps the tagging in 2.3 is a good answer, I certainly enjoy tags in flickr and del.icio.us.
But all is not rosy, I tried to delete a new test post and got the message “You are not allowed to delete this post.” — not cool. So I set it to “draft” and will wait.
Looks like 2.3 has some quirks.