Apple has been talking about OS X Lion for some time already, of course, but it’s now filled in most if not all of the remaining key details at WWDC. Dubbed a “major release” with over 250 new features, the OS adds things like a slew of new multi-touch gestures and full-screen apps (including iPhoto, iMovie, Safari, etc.), plus the all new Mission Control, which unifies Expose and Spaces, and the iOS-esque Launchpad application launcher. It also includes a new system-wide Resume feature that lets you pick up exactly where you left off, a new auto-save feature that automatically saves different versions of documents, the new AirDrop peer-to-peer file-sharing system, and a brand new version of Mail that finally includes a conversation view.

The big news revealed today, however, is that the OS will now only be available in the Mac App Store as a 4GB download — which installs in place, no reboots — and that it will run you just $29.99 for all of your authorized Macs. It will be available sometime in July, but developers can get the latest preview release today.