I really wish Windows apps had more consistency in their fullscreen behavior. Sometimes they bring up the taskbar, sometimes they behave like normal windows, sometimes they don't even let you scroll off the side anyway. This last behavior is most annoying in strategy games when I'm scrolling the map and have to use the arrow keys for one of the directions, but equally annoying in some FPS games where the cursor scrolls off the side but still behaves normally until you click to fire and then the application loses focus.
I'm not even a Mac user but I would take a consistent but irritating behavior over an inconsistent and also irritating behavior. It's more inconvenient to have to disable my second monitor while I'm playing a game.
I've found that Dota2 has a very good implementation for multi-monitor behavior. When you're in a menu or waiting to find a game in queue, you can move your mouse off to the second monitor, and click on something and the game will minimize. When you're in game, the game captures the cursor, and doesn't let it move to the second screen unless you alt-tab out of the game window.
I'm not even a Mac user but I would take a consistent but irritating behavior over an inconsistent and also irritating behavior. It's more inconvenient to have to disable my second monitor while I'm playing a game.