Without a doubt, my favorite new feature is comment moderation keyboard shortcuts. It has bothered me for a long time that comment moderation was such a tedious chore. Keyboard shortcuts (and the new inline reply) make it significantly less of a chore.
The new customizable post screen is a close second. I love that I can completely hide away all of the stuff that I don’t use (by clicking the “Screen Options” tab), and keep tags and categories on the right, while increasing the size of my post content box.
The new Publish module on the post screen is a personal treasure of mine. I took point on that, with some excellent design guidance from Jane Wells. It moves the “Save Draft” and “Publish” buttons far apart (a common complaint was that their proximity lead to accidental premature publishing. The Preview button now shows you the most recent changes to your post… not just the last saved version. Additionally, you can preview changes on published posts without those changes being shown publicly (until you’re ready). The Visibility section is new, and contains the functionality of private posts, password-protected posts, and sticky posts. Future posting is a lot more clear now. When you edit the time stamp to point to a future date, the “Publish” button becomes “Schedule.” All these changes were made to make your Publish module function predictably, so that you’re never wondering what happens when you click something.
The new menu system is great. I operate it in folded mode, so that my content can really take center stage.