First thing's first: Don't, please, say that the issue is Windows 8; I know like 80% of you guys are probably hating on it, but hey, I like it.
So, for most everything (including Steam chat windows) I can have a metro app full-screen on a second monitor without any problems.
Edit: This also applies when the 'docked' app is on the larger side of the screen.
But when I click on the Steam Friends window, or the main Steam window, it automatically switches my second monitor to the Desktop. I haven't had this happen with any other apps or programs, and I can't figure out what causes it.
It happens when Steam is in full screen or windowed mode. It happened before the "Big Picture" update was pushed through as well, in case you were wondering.
Perhaps weirdest of all is that it only does it once I close and re-open all my steam windows. So if I start Steam and simply minimize the main Steam window without ever closing it, I never get the issue. The moment I close it and then open it from the tray icon, I can't click on the Steam window without it minimizing my apps on my second monitor.https://dl.dropbox.com/u/4780235/Before%20Click.pnghttps://dl.dropbox.com/u/4780235/After%20Click.png
The only differences between these two screenshots is that I clicked on the Steam window. Seriously, that's it.