Everything. Literally everything. It uses a WYSIWYG input which completely breaks old.reddit formatting inputs, it constantly causes confusion as to what comment you are replying to so you get people replying to the wrong person or replying to the main post, it looks cartoony and blatantly obvious you're on a social website if you're trying to reddit on the sly, It doesn't like browser windows that aren't open full screen if you're trying to reddit on the sly, it severely restricts how much information you can have in the sidebar, etc.