A million things, like missing functionalities from the new extensions api (meaning no Pentadactyl), no good way to manage keyboard shortcuts, having to disable many google integrations after installation, no way to disable "do not track" if using built-in tracker blocking, no sidebars a la Vivaldi, buggy rendering (e.g. transitions animating elements using css transforms), unexplained slowdowns, lack of proper tab isolation (one slow/crashed tab takes the whole browser with it), etc. I could rant all day.