As we've noted in our past previews, Mac OS X Lion will bring many of the best elements of iOS to the Mac.
Some of the highlights of Mac OS X Lion include:
Multitouch gestures
Full screen applications
Mission Control
Auto Save and Versioning
Launchpad
Auto Resume of apps
Mac App Store -- now with in-app purchase
AirDrop - P2P Wi-Fi networks
Newly designed Apple Mail
Better Search
Apple is clearly pricing Mac OS X in such a way that most users will probably upgrade. Rather than selling the OS in stores, it will only be available in the Mac App Store. The 4 GB download might be excessive for some, but having installed every past version of the Lion Developer Preview, I can say it isn't terrible. Moreover, this ensures that users can install Mac OS X Lion on every Mac they own without having to buy another license.
Mac OS X Server will now be an in-app purchase rather than a separate version.