Known changes so far:
1. The writing box completely goes away, even if you're already drafting something. By default it only shows a timeline. You need to click on 'Publish' to get it back (which sounds more like an immediate action than something that gets you a box to write?).
2. As discussed elsewhere: if you're not logged in, the [[Create Account]] button might send you directly to https://joinmastodon.org instead of giving you an option to create a local account (!).