If you have a Magic Mouse or Trackpad, you'll be able to use gestures and see that it does much more than SL.
If someone is using an older Apple Mouse, or a non-Apple Mouse, they aren't going to see all the cool things Lion can do.
I've been using full screen Mail, browser, other apps, etc. and switching between them is almost instant using gestures. That alone is worth upgrading for me, but the new updated Mail app is also terrific. Then there are a ton of little changes, a few I do not like, but most of them you can change in the defaults under System Preferences to whatever you prefer. I haven't gotten used to the inverted scrolling and changed it back to normal, despite having an iPad.
Mission Control, for me, makes using Spaces easier and combining Spaces and Expose was a great idea. It makes for a faster overall experience.