Here's some of the more little changes that I've seen.
iCloud documents.
New dock.
Incremental volume adjustments (hold opt+shift when changing volume)
Notification Center.
Non-grouped Expose ala 10.5 Leopard.
Lion
Mountain Lion
Turn off Resume and Auto Save.
Dictation
Finder copy/move files progress bar in icon.
Sections in Finder sidebar can be rearranged.
Launchpad search.
Background software updates, automatic security updates, etc.
Safari-style inline search in Mail messages.
Preview annotation tools.
Expanding scrollbars over hover.
What? It's definitely more of a service pack. 80% of the features on the Apple site are stuff like "new groups column in address book". I mean, come on. I don't know how they're handling Safari updates but you shouldn't have to pay $20 just to have your default internet browser get an update, ffs.
Who said you had to pay $20 to upgrade your default browser? Safari 6 is coming for Lion as well.