Reason #10: Do you use iOS? You’ll feel right at home
Reason #9: AirDrop – the future of collaboration?
Reason #8: Wikipedia searching is now much faster
Reason #7: QuickTime is now more of a movie editor, less of a player
Reason #6: You won’t need an install disc ever, ever again
Reason #5: It’s rock-solid
Reason #4: Free HD Video Chatting
Reason #3: FileVault now encrypts external drives
Reason #2: iChat, your next instant messaging client?
Reason #1: You use a Mac, you need to have the newest thing!

from: http://www.redmondpie.com/10-reasons-why-you-should-upgrade-to-os-x-lion/