In my opinion, the best function of WordPress is the edit shortcut when you visit your site. This is extremely helpful in the final stages of production because you can view your site, catch a typo or another minor problem, and hit edit. This takes you back to that page or post on that dashboard. It eliminates several steps that you would have to do without this shortcut making editing fast and easy.
The worst function of WordPress is not being able to save a draft of a page. Being a student, I would work on the blog at odd times, sometimes in between classes. Even though a page I was working on was not ready to be viewed by the public, I would have to Publish it just to save my progress. I am a bit of a perfectionist so I found this frustrating to Publish and incomplete version.