--If you have Boot Camp, supposedly you can now open Mac files from Windows. Yay! :D (I would assume Mac OS X has a way of keeping Windows viruses from spreading into the Mac system, however?)
--signal strength for Wi-Fi. Now we will know which network to choose, rather than just choose one and hope for the best!
--put-back command in Trash. Like Windows (yes, Windows had it first...sort of... :P), SL will now feature a put-back option to restore deleted files.
--select correct text bodies in PDFs. When we are selecting text in a PDF, Leopard doesn't select just the body we want. But SL does, by means of "advanced algorithms" so yay Apple for doing that. :D
--windows minimize into dock icon (system prefs>dock>change those settings) so now instead of making your Mac dock go WAY OFF THE SCREEN!!!!! (yes, you know who you are and you know what I mean!!! :P), now the windows just minimize into the app icon. Windows 7 copies this feature.
--smart eject (tells you why you can eject drive). So when you can't eject your flash drive, it will tell you why so you know what you have to do to eject it.
--screen recording w/ QuickTime X. QuickTime X looks awesome and is awesome and you can record screenshots, edit videos and....yeah. I would like to see the QuickTime X theme used throughout SL, just as an alternative option to the white color. :D I really like the black QuickTime X look.
--new backgrounds/screensavers. Apple techs added some awesome looking backgrounds/screensavers. Check out the new backgrounds Macs are shipping with on Apple.com.
Besides those cool features, there are a few more, like black menus for easier viewing.
Basically, SL does an improvement of performance and productivity. For 10 bucks (if you bought a Mac after June 8th) it is a great deal.