You can label this a new feature, but it just happens to be one that reduces functionality and usability of the system for use cases that include multiple monitors. Had Apple implemented full-screen mode such that apps stayed on the same virtual desktop as they were in windowed mode and made it user-definable on a per-app basis whether to blank secondary displays, it would have been quite okay. Moving an app onto a new virtual desktop is iOS methodology that, IMO, has no place on a desktop operating system. Forced blanking of secondary displays? User-definable would be a feature, but forced? That's horrifyingly bad design.
The guy in this thread who has 6 displays should be able to run 6 simultaneous full-screen apps, one on each display. As it is, the guy who thinks, "Shiny! I can string a bunch of Thunderbolt displays on my MacBook Pro", will be mighty disappointed to find himself the owner of 2 grand worth of dark linen the first time he puts Safari into full-screen mode on his laptop display. Users shouldn't be forced to find 3rd-party apps to enjoy sensible full-screen implementations.
Apple would have done well to advertise this new feature as a cool, retro return to 1990. Back then, task switching between full-screen apps was all the rage. DOS and DESQview did a great job of it. Alas, it's 2012 and I expect better from Apple. Hiding complexity is always a good thing. Taking choice/power away from the user ... not so much.