When you switch between open applications by clicking their icon on the dock or using Command-Tab, Spaces switches you to the space where that application has an open window.

If that application has windows open in more than one space, it alternates between them. So, if spaces 1 and 3 contain open Finder windows, and spaces 2 and 4 contain open Terminal windows, switching back and forth between the two will show you all four spaces in a consistent, predictable order.

Except when it doesn’t. Currently, I’ve got a set of windows Spaced out where repeatedly pressing Command-Tab takes me on a very peculiar tour: 1, 5, 2, 4, 1, 5, 1, 5, 1, 4, 2, 4, 1, 4, 2, 5, 1, 5, 1, 5, 2, 5, 1, 5, 1, 5, 1, 5, 2, 5, 1, 4, …

Wish list for Spaces:

• visual indication of current space (background color, etc)
• named spaces (#3 is less useful than "build monkeys")
• ability to isolate desktop icons to a single space
• consistent switching

[Update: I killed it! I added a second Terminal window to one of my spaces and started rapidly hitting Command-Tab to see if it changed the behavior, and after a few cycles, Spaces went kablooie. I lost Spaces, Dashboard, Dock, Exposé, and Command-Tab. Fortunately I landed in a space that had a Terminal window, because “Force Quit” doesn’t include an option to restart the Dock (which is the parent app for all of the above).]