Peter Cohen, writing for iMore:
But when OS X Lion came out in 2011, Apple backslid when they introduced full-screen apps, which would cause one monitor to display a pattern while the other one held the app. And using Spaces since Lion - especially with multiple monitors - was an exercise in teeth-grinding frustration. Apple’s changing that in Mavericks, making multiple displays finally work the way they’re supposed to.
The setup at my day job consists of dual 24" Apple monitors attached to a Mac Pro. Ever since OS X Lion, I have wanted to punch a fist through one of the displays for reasons Cohen mentions. The one feature I find even better than fullscreen apps on each display is the implementation of independent menu bars.
Until now, the experience with multiple displays has been a nightmare. I for one am very happy to see Apple finally get this right.