Whereas in the Spaces view in Snow Leopard I could move a window to a different space AND a different monitor with a quick drag, Mission Control doesn't allow this - I have to first move the window to the correct "Desktop", then exit MC and drag it to the correct monitor.
Full Screen Apps are similarly useless on multiple monitors - regardless of which monitor the window originates on, clicking the full screen button returns it to whatever display is designated "primary". Even worse, while you're in full screen on one display, the others become useless as it blocks them out with Apple's new favorite canvas texture. Why?
At first I thought all this was intentional and that Apple was leaving multi-monitor users in the dust to focus on Macs that are very close to iOS devices (11" Macbook Air comes to mind), but today they also announced a new Cinema Display that lets you daisy chain two monitors off a single Thunderbolt port. Clearly it's still a mode of operation they intend to support for some time - so why have they crippled it so horribly in this release?