The just-released Mac OS X Lion has a lot of killer features, but so far, for me, the most killer is the ability to easily toggle any app that supports it into a distraction-free, full-screen mode. Apple's marketing page for the feature talks about it in terms of making apps more immersive and "gorgeous", and that's certainly one nice aspect. But what I most appreciate about going full-screen is how it helps me keep my work focused.

In the last couple of years a whole subgenre of new word processing apps have come out designed to help writers focus on writing by blocking out everything else, by entering a full-screen mode where your text — not email, not Twitter, not FarmVille — is the only visible thing. If you need to look something up or go track down a photo or some other asset that's fine, but you have to switch contexts in order to do it.

The basic principle here is that we humans are far less capable of multi-tasking than we think we are, and some aspects of computing environments intended to help us be more productive actually turn computers into productivity-killing distraction machines. And so these new apps — WriteRoom pioneered the genre, but iA Writer and Byword may be its best current examples — help us be productive by tricking us into believing our computers are good for only one thing at a time, just like they were in the good old DOS days.

So to appreciate Lion's full-screen apps, imagine that same concept, but applied to everything on an entire modern desktop computer. Some word processors and photography apps have had full-screen modes for ages, sure, but what about email? (I always get distracted writing emails, one reason why it takes me forever to respond to them.) Or making Keynote presentations?

And if focus is great for creating content, but what about consuming it? What about reading an eBook? Reading a PDF book in Preview in Lion, in full-screen mode, is a lovely experience, and makes me really look forward to whenever iBooks finally makes it to Mac OS X.

Of course, focus is at least as much about the things you're ignoring than the ones you're not. Another great benefit of putting apps into full-screen is not having to see or care about them when you're not looking at them. I like to keep Terminal (which I use a lot when developing Rails apps, to run test suites or the Rails server) and iTunes in full-screen mode, outside of any space I'm working in. I can still switch into these apps easily if I need to, and they can still get my attention via the Dock, but otherwise they just fade into the background where I can most easily ignore them.

This is what makes Lion's full-screen apps so nice. They're not just bigger windows on a normal desktop — each one is a whole desktop Space in itself, with nice, clean, clear contextual separation from everything else on your Mac. Where's your email? It's in the email space. Browsing is in the browsing space, calendars are in the calendar space, so on and so forth.

(This, by the way, is one solid argument in favor of some of the absurdly skeuomorphic new UI designs in Lion for apps like iCal or Address Book. iCal's new look, where it resembles a leatherbound desk calendar, seems silly, but it screams calendar! in a way that makes it very, very easy to spot, especially in a zoomed-out view like Mission Control.)

I've always used different Spaces to separate windows/apps into different working canvases or contexts, and it's worked out okay. But it has been work remembering which Spaces had which apps in them, or having to manually drag windows into the right places to keep everything organized, or having to keep around more desktops than I needed at a given moment. In Leopard and Snow Leopard there's always a grid of at least 2 Spaces, where you could add or remove whole columns or rows but not single desktops. I usually stuck with a 2×2 grid — four whole desktops — and typically would only use the top row. But since changing the Spaces layout meant a visit to System Preferences, it was always better to keep a couple of empties around just in case.

Lion's new Mission Control UI fixes Spaces by doing away with the grid layout, where another space can be spatially up, down, left, or right from wherever you're at, to a single row of apps and desktops. There are no empty, unused desktops sitting around, and adding a new one is as simple as dragging a window onto a big 'plus' button in the upper right corner. The mix of full-screen apps and only as many desktops as you need makes it easy to understand what's where, so navigating and switching contexts is much simpler and nicer.