While I’ve actively avoided most of the new features and functionality in OS X Lion as part of my Focused MacBook Air effort I’m a huge fan of the 2-finger swipe between pages.
Right now, I see it as the only part of Apple’s Bring-iOS-to-MacOS UI effort that feels natural, comfortable, and OMG-WHY-DIDN’T-THEY-DO-THIS-SOONER!
Not having it in the Finder, Mail, and on all my other Macs feels as un-natural as not having Quicksilver installed. And I’ve only had it for 10 days.