The thing to reverse would have been the "switch to new workspace when full-screening" - or at the very least make it optional. Certainly not respond "that's a feature" and close as "working as expected" when thousands complain.
I failed to see the value in that feature even with a single screen. An action that used to happen instantaneously now took 1-2s. and a dizzying sliding animation. (Many a flow was lost to toggling full-screen by accident - whereas previously you could toggle/toggle back immediately without losing your mental state - surely you appreciate that as a developer?).
Full screen was an effort to make OS X more usable on small displays - recall that the 11" MacBook Air had just shipped. It didn't make much sense for media playback apps to adopt the system full screen mode, especially as it was in 10.7-8.
I would have loved to enable a system mode where full screen windows could coexist in the same workspace as unrelated windows, but this would have been a new feature, not something we could have achieved by reversing anything. And eventually Apple did enable a new mode, which was what shipped in 10.9.
Oh, and if you filed a bug, then whoever closed it as "working as expected" made a mistake. There was a (heavily duped) bug tracking the uselessness of secondary displays in FS, and it was closed when 10.9 shipped. I may even have been the one to close it, I don't remember.
This doesn't make much sense, does it? Obviously apps adopt the system implementation. The problem the OP is talking about is that the system implementation became pretty weird, broke some apps that used to work just fine (especially lots of xquartz ones), and thought that the best use for your extra monitors was to just display a dark gray pattern.
Personally, I learned to live with it (sigh, uncheck "displays have separate spaces"), but it does seem like a good example of Apple shoving a half-baked idea out the door.
I can't specifically recall, but I think the same was true of full screening video content from Safari (perhaps technically a "plugin feature", but maybe html5 video was around?), but QuickTime was 100% an apple regression.
Full screen in 10.7 and 10.8 did render secondary displays useless. But it was a new feature in 10.7, so there was nothing to reverse. You can't take Safari, Mail, etc. full screen in 10.6.
Perhaps there was an app you used that switched from a custom full screen implementation to the system one, and so regressed on secondary displays?