Mavericks: Window across multiple monitors?
Quoting from John Siracusa’s review of Mavericks (emphasis mine):


  If you prefer the old behavior, uncheck the “Displays have separate Spaces” checkbox in the Mission Control preference pane. Doing so will also restore the ability to have windows that span more than one display.




So yes, it’s just a setting. It’s been put in the Mission Control pane, rather than Displays, because the effect of this change is to allow you to have different full screen apps on different displays.

The page of the review that I linked to explains the change in more detail (and specifically discusses the window-across-multiple-displays behaviour that you want).