Ok I think I've figured it out.
I'm using it wrong. I'm still thinking in terms of how things worked in SL.
According to this video here . . .
http://www.youtube.com/watch?v=bG9l1l2cZ-E&feature=related
There's really no need to assign apps to a space. In fact, doing so seems to defeat the purpose of Mission Control. Mission Control already groups all of your windows together, and these groups are separated by app. So in effect, each app does have its own space in Mission Control, but when you're not in Mission Control it's all jumbled in one Window. I'm not sure if I like this behaviour. However, if I want to focus on an app I can go full-screen with it and it'll set it off to the side, sort of in a "focus area." Now *that* I can understand.
Of course, if you go full screen with multiple windows of the same app, these windows will be shown separately in this "focus area."
In light of all this I'm not sure what the point is of assigning anything to its own desktop. Unless you are on that app's assigned desktop, Mission Control won't expose any of it in the usual manner.
Do note that in terms of focus, if you have multiple windows open for the same app and you do an expose, then all you'll get is an expose of just those windows. That's a nice touch.
No wonder Apple made it a little more difficult to assign apps to desktops. It seems Apple is encouraging users to do everything on the same desktop and allow Mission Control to handle the grouping/organizing when you need it. And if you need to focus on something without any app jumble peeking out behind it, then go full screen and you can access that window on the side.
If you want to maintain focus on a particular app window - you're working on a paper, etc. - and you don't want to go full-screen and refer to the "focus area", then you can simply enlarge the window. You can do it the Apple way (I admit, not the greatest in this case) or use a utility like Right Zoom:
http://www.macupdate.com/app/mac/30591/right-zoom
So . . . unless you work with an app for which you open a lot of windows (i.e., multiple Pages documents at a time, multiple Safari windows, etc.) then it isn't really worth it to use multiple desktops.
So I kind of get it now. I'll use it the "intended" way for a while and see how things pan out.