I’ve never used Spaces on Mac OS X. I’ve seen plenty of accounts from others that do, and I’d considered that they could probably fit well into my workflow. I just never put the time in to learn them and get comfortable integrating them into my daily computer use. It was one of those features like Command-Tab that seemed useful, but which I never got into the habit of actually using.
Then Lion came along. I tried out fullscreen with some of the apps that support it, like Sparrow, and my favorite associated feature was immediately gestures. I love being able to swipe between a fullscreen app and my main workspace. I can keep my email or my music off to the side, to slide in when gestured or clicked and to be easily dismissed with another quick swipe. Swiping back and forth between fullscreen apps and different desktop spaces is elegant and effortless.
With fullscreen apps and Mission Control, Apple has made multiple desktops easy to use, easy to understand, and most crucially for me, easy to get started with. While not everyone likes fullscreen apps or gestures, to me, this is an example of exactly what Apple is trying to accomplish with Lion: creating a system where the average user immediately knows what to do and feels comfortable doing it; where interactions with the computer feel natural and automatic.
It is this guiding theme, not any specific feature, that represents the “iOSification” of Mac OS X.