I really dislike the new integrated system status because it increased the number of clicks required for almost all operations - worst is connecting to a WiFi network which now requires 5 clicks all over the screen (up from 2 in a corner). Bluetooth is puzzlingly only displayed when a device is connected, so you have to take the long path through settings to change visibility, add a new device, or even send files (!!). Empathy status cannot be changed from gnome-shell anymore, but Empathy still runs in the background when you close it. And so on..
They also removed the option to have (generic) icons in menus and on buttons. How is that a usability improvement?!?!
Photos, Documents, and (to some extent) Music still display their respective files as massive jumbles.
Maps is nice, but, again, the devs made some really weird "usability" choices. After you search for something, the map pans to the selected POI. The closer the POI is to your current view, the longer it takes - a one-block pan takes 11 seconds and is uninterruptible. To top it all of, the map zooms out a few levels when it's done panning .
Header bars are nice, but you now need 2 clicks to minimize the window. The new annimations and general polish are nice too.