When using dual monitors and you want to full screen an app, the primary monitor will get the app full screen, and the secondary monitor will get a grey frabric-y texture (screenshot'd), I've discovered no way to make the secondary monitor useful in any way when in full screen mode. EDIT: Not sure where that screenshot went, here's a direct link to it http://cl.ly/3K3k3W2P0J2B2T450o3K
Also spaces still isn't monitor independent (both monitors are treated as a single space instead of being able to switch to different spaces on each monitor)
Preview now has a magnifying loupe which I'm fairly sure is not present in 10.6.
Same thing here.
My dual monitor setup in fullscreen mode only uses one display.