One of the more interesting changes in Mavericks is the way it now handles multiple screens and what was called “Spaces”
(is it still even called that?)(yes, yes it is).
Previously if I had a second monitor attached to my MacBook Pro, both screens always changed when I switched Spaces. It was like each Space each had two monitors attached to it (the one on the the laptop and the attached big monitor).
But now, each screen has its own set of spaces. These are independently switchable. It took me a minute or two to suss this out. Mavericks gave my laptop screen (for the purposes of a two monitor desktop, the secondary screen) only one space by default, and put the default four spaces only on the large, external monitor (which in my multi-monitor setup, I have designated as the primary display). So now instead of four spaces each with two monitors I’ve got five spaces, four on the external monitor and one on the laptop.
I also suspect that’s why when I installed Mavericks it took off my customised wallpapers and applied its default “surf” image.
Update: it’s controllable in the System Prefs – Mission Control – Displays have separate Spaces