Developers may be giddy over Mac OS X Lion's more advanced features - which pull in multitouch and full-screen app behaviors from iOS onto the desktop platform - but it's AirDrop that is likely to make the biggest impact on everyday users. Apple's new wireless file transfer system, AirDrop is all about making impromptu shares with the users around you, and doing it in an incredible straightforward way.
AirDrop is added as a new icon in the Finder sidebar, and when clicked OS X Lion automatically scans for nearby users also using AirDrop. If they're in your Address Book then you'll see profile photos, too; otherwise it will be a MacBook name.
Files to be shared - whether music tracks, documents, images or something else - can be dragged from the Finder window straight onto the AirDrop contact's name and, after the transfer request has been accepted by that person, automatically shuffle over and straight into the person's Downloads folder. Closing the Finder window automatically turns off AirDrop, so that you won't inadvertently show up in other people's search results.
What remains to be seen is what happens to existing apps using the AirDrop name: there's already an iOS game called AirDrop [iTunes link] as well as a company making a podcast downloading app [iTunes link].