I've given Apple a pass on this one up to now but I really think they need to sort it out now.
They radically changed support for multiple monitor and full screen apps in Lion.
The issue is that if you go full screen on an app when you have multiple monitors, the app goes full screen on one monitor and you get just the desktop on the other. The second monitor is virtually a waste of space. If the full screen app supports floating panels (such as an Inspector panel), you can drag the floating panels onto the second monitor, but the main application is locked to the screen you went full screen on.
You can't access or view a second application on the second monitor.
What a criminal waste of valuable screen real estate.
I suppose the thinking is that "full screen mode" for an app is intended to allow you to 'focus' on that particular app. It works great on an 11" MacBook Air, but on 2 x 27" Apple Cinema Displays, not so much.
You can switch to another app using ⌘ SPACE, but the full screen app disappears into another desktop space - crazy!
Apple did make some changes to the default behaviour in Mountain Lion in that when you go full screen on an app, it goes full screen on the monitor you went full screen on, and not the primary display, but it's still pants!
It gets worse...
I was driven to write this blog post this morning following a quick Remote Desktop session. I'd had my Mac mini connected to one of my 27" Tunderbolt displays temporarily, but decided to return back to my usual set up of having both 27" displays connected to the MAcBook Pro with Retina display (rMBP).
After all, I could access the Mac mini via Remote Desktop and blow it up full size.
I'd been used to using the Mac mini at the full resolution of the Apple Cinema Display - in all it's 2560 x 1440 pixelly goodness.
Firstly, I connected via Remote Desktop to the Mac mini from the rMBP, with the Mac mini still connected to the Thunderbolt display. The Remote Desktop session connected and displayed the Mac mini at the same resolution of the connected monitor - 2560 x 1440, excellent.
I reconnected both Thunderbolt displays to the rMBP, and rebooted the Mac mini without the Thunderbolt display attached.
On the rMBP, I loaded up Remote Desktop, connected to the Mac mini and now the maximum resolution I can get from the Mac mini is 1680 x 1050.
If I go full screen on the Remote Desktop session to the Mac mini, I get a 1680 x 1050 desktop on one Thunderbolt display with a huge thick black border around it, and a completely black screen on the other Thunderbolt display.
So out of the available 7,372,800 pixels available to me, I get to use 1,764,000 or just 23.93% of the available pixels.
That's using the rMBP in clamshell mode - not using the built in Retina display.
If I open up the rMBP and switch it to the highest supported resolution of approx. 1920 x 1200 (it can actually go much higher), now I have a teeny 1680 x 1050 window and two black screens - out of the available 9,676,800 pixels available to me, I get to use 1,764,000 or just 18.23% of the available pixels.
So please fix the ludicrous use of full screen apps on multiple monitors in the next version of OSX, and tweak Remote Desktop to support higher resolutions when connecting to machines running heedlessly.
Windows Remote Desktop access has done this for years!