One of the touted features in Mavericks is better multi-monitor support. And at some levels, that's true. Unfortunately, my overall experience is that things are worse, not better, than they were before—especially if you don't use full screen mode often (or at all).
So what's good? To me, these are good changes in multiple display support:
Full screen app support: Sending an app to full screen mode (optionally) no longer kills other attached displays. Finally I can have one full screen app, and still use the other two displays in my setup to work with other apps. This works great, for instance, with an Apple keynote going in full screen on the main 27" iMac, and my other two displays filed with "real" work windows.
Menubar and Dock on the active display: I'm still not 100% convinced about this one, but it is nice not having to jump back to the main display to reach the menu bar or Dock. See the minuses below, though, for a couple oddities about this.
Unfortunately, that's my full list of plusses in Mavericks regarding multiple display support. So what'd they get wrong? A lot more…
Windows can't span multiple displays. If you drag a window partially across two displays, whether on purpose (really wide window) or accident (dragged it a bit too far), the smaller side of the window simply vanishes; it's chopped off at the screen boundary. I don't often use a split window on purpose, but I do occasionally drag a window too far—and if I want to see what's in the missing bit, I have to then drag it back.
You can disable this behavior, but only by turning off the most-useful "full screen apps get their own space" feature. That's a tough tradeoff.
Many apps open on the active display: When you position an app at a certain spot on a certain screen, you generally do that because you want the app there. Prior to Mavericks, quitting and relaunching an app would open it right where I left it. In Mavericks, that's not always the case. Mail seems to work, opening on the display where I quit the program.
But this isn't the case for everything, and not just for third party apps. Terminal, for instance, refuses to reopen where closed, and launches on whichever display is active (i.e. has the menu bar). Same for YoruFukurou, BusyCal, Automator, Calendar, etc. It is an incredible pain to have to remember where I want the window before I launch the app—or to move it after launching. Mountain Lion handled these apps perfectly.
Active screen is based on clicking: Mavericks displays a menu bar on each display, but the menu won't be active until you click on that display. I'd love to see an option where the menu bar's focus follows the mouse, and becomes active as soon as my mouse enters a given display.
The Dock is unpredictable: I keep my Dock hidden at bottom center. If I'm working on my main display (the 27" iMac's screen, defined to hold the menu bar in System Preferences), the Dock pops up right away. If I go to one of the other two displays, it's uncertain what will happen when I summon the Dock.
If I've merely moved the mouse to another display, the Dock will not show when I mouse down to bottom center of the screen. I have to first activate the display (click required), and then, it'll work…sometimes. Sometimes it works, but only after I hover five or more seconds at screen bottom. And sometimes, it just doesn't come up at all, and I have to click into and out of a few apps to make it work.
You can argue that "focus follows mouse" may not make sense for changing the active menu bar. But it definitely makes sense for the mouse-activated Dock. If my mouse is on a given display, then that's where the Dock should appear—regardless of the position of the active menu bar. (I'd argue that if you activate the Dock on a given display, the menu should instantly activate on that same display.)
No options for the multiple menu bar: I'd really love to be able to turn this off—there are times I don't want or need the menu bar (and Dock, for that matter) to follow my actions on another display. I'd love to be able to work with it the way I did in Mountain Lion—but in a way that I could quickly toggle the feature on and off as needed.
Of the negatives, the ones I find most troublesome are the inability to view a split-display window and apps opening on the active display, instead of their last-opened location. Hopefully the first can be fixed with a new feature addition in a future release, and the second is some sort of bug that'll be squashed in time.