Fixing multi-monitor workspaces on Gnome 3.x
Gnome 3.x changes the way multiple desktops work when using multiple monitors. On Gnome 3.x, by default, only the primary monitor can change workspaces. The remaining monitors have fixed workspaces. If you want the old behaviour back, open a console and run this:
gsettings set org.gnome.shell.overrides workspaces-only-on-primary false
And you're all set up.