Literally everything. I've used it for a grand total of about 3 minutes but from what I remember:

* Defaults to some stupid instagram wall-like view where every link takes half the page

* The whole thing is now centered in a tiny slice of the screen

* Threads open in some weird modal mode where accidentally clicking outside of the content takes you to the previous page

* Opening a direct link on a phone loads like 5 comments, then it continues with endless irrelevant content

* You can't open random subreddits that have been set as NSFW on the phone because "This community is 18+, please open it in the app"

Every time I had the displeasure of interacting with the new design there was some upsetting regression compared to old Reddit.