On an Edit Post page, if you're typing a tag and the suggested entries pop up, scrolling through those entries with a keyboard and pressing 'Enter' on the desired entry doesn't add the full tag, only what was manually typed thus far. It works if you click on the entry with your mouse, but not by pressing 'Enter'.
It's super annoying and now obnoxiously slows down tagging. I tried deactivating all my plugins and the problem still occurred.
Everything else about the new WordPress is just grand though :)