First of all, multiple displays in Mavericks can work independently. Each monitor can have its own menu bar. Let me repeat that so it sinks in: every monitor can have its own menu bar. That's never happened in OS X before, without installing third-party software.
What does this mean in practical terms? It means you can have two applications running on two separate displays, each one with its own independent menu bar. That means a lot less moving the mouse back and forth: up until now, you've had move the cursor back to your "primary" display to access the menu bar for the app running on a second display. Inelegant and confusing.
The Dock is available in any screen to screen - so if you move your cursor to the bottom of the screen (or wherever you've designated the Dock to appear), the Dock will be available.
Full-screen app support finally works the way it's supposed to, as well. Go full-screen on one monitor and the app will, predictably, take over the display. But the second monitor is unencumbered. You can go full-screen with another app on that one, or just use it in regular windowed mode if you prefer.