On the lock and login screens, The clock and interactive UI elements are now only shown on one screen at a time when using a multi-screen setup; they fade out on screens without the pointer or keyboard focus, leaving those screens free to display pretty wallpapers. (Yifan Zhu, link 1 and link 2)

Best and smartest implementation ever

System Settings' Display Configuration page gained some UI Improvements; now the screen arrangement view is hidden when there's only one screen, and with more that one, there's a big obvious screen chooser at the top of the page to make it clear which screen is selected, and when there are any disabled but connected screens.

I don't know how I feel about this one. I feel like the top chooser is redundant since you can already click on the monitors icons.