Lion contains over 250 new features including multi-touch gestures, Mission Control, LaunchPad, full-screen apps and a completely redesigned Mail app. Additional new features include:
Resume, which conveniently brings your apps back exactly how you left them when you restart your Mac or quit and relaunch an app;
Auto Save, which automatically and continuously saves your documents as you work;
Versions, which automatically records the history of your document as you create it, and gives you an easy way to browse, revert and even copy and paste from previous versions; and
AirDrop, which finds nearby Macs and automatically sets up a peer-to-peer wireless connection to make transferring files quick and easy.
Lion marks the first time that Apple has distributed a whole number OS upgrade exclusively online. That's right, Lion isn't available on optical media, you can only get it from the Mac App Store as a 4GB digital download -- at least today.
If you don't have broadband access you can download Lion at Apple retail stores. If you administer a lot of Macs Apple will offer Lion on a USB flash drive through the Apple Store for $69 (US) in August. The third-generation MacBook Air and new Mac mini will both ship with Lion pre-installed and on a USB flash drive.
If you plan on upgrading to Lion, stop. Take a deep breath and do a little preparation before making the move. If you're using custom, vertical or other special software to make a living, be sure to check that all of your apps are Lion compatible with their developers. And definitely make a full, bootable backup of your Mac to an external drive and verify that it boots and that all your data is there.
Mashable's Lion prep guide has some other useful tips for preparing for the big cat, including tips on how to clear off disk space (I've been using Disk Radar) and how to find PowerPC apps that are no longer compatible with Lion since Apple officially dropped support for Rosetta.
RoaringApps.com is maintaining a list of Lion compatible apps that is extremely helpful for those planning on upgrading. For example, Verizon's VZAccess Manager software for its USB modems isn't Lion compatible.
Update: MacRumors notes that Apple's new hardware (today's new Airs and minis) support Lion's Internet Recovery feature which allows users to install Lion onto blank/new hard drives in the event of hard drive upgrade or replacement.
If your Mac problem is a little less common — your hard drive has failed or you’ve installed a hard drive without OS X, for example — Internet Recovery takes over automatically. It downloads and starts Lion Recovery directly from Apple servers over a broadband Internet connection. And your Mac has access to the same Lion Recovery features online. Internet Recovery is built into every newly-released Mac starting with the Mac mini and MacBook Air.