The problem that remains in Mountain Lion is that you can only use 1 display at a time when in full-screen mode. It's better than Lion in that you will be able

The problem that remains in Mountain Lion is that you can only use 1 display at a time when in full-screen mode. It's better than Lion in that you will be able to choose which of your displays is used to go into full-screen mode, rather than always just using the 'main' display. Disclaimer: I haven't actually tested in Mountain Lion, but this is the only consistent explanation I've been able to gather from careful reading of people's complaints and Apple's description of the feature change.