…it’s the next step in a progression that began with iTunes and has slowly spread to the Finder, iLife, and Xcode: the single window UI as standard. In Lion it’s not just a single window, it can be a full-screen window so that the windows of other apps can’t confuse you. The window itself ceases to exist, the desktop ceases to exist, and the UI becomes mono-tasked.
It’s weird because with today’s larger screens multiple windows work better than ever.
Update (2010-10-24): Lukas Mathis:
While I agree that the window management system we currently have is broken, simply doing away with it altogether is not solving the problem. It’s capitulating.