Lion has a lot of nice little features, but it's not as large of an upgrade as Leopard or Tiger (even though Apple is billing it as such with the 250+ features marketing jargon).
However, the price is right, so to upgrade is a no brainer.
Things I Like
⁃ Dashboard being over to left
⁃ New gestures (although some of them are rather difficult to remember)
⁃ Versions
⁃ Automatic Save
⁃ iCal's "Quick Event" (aka Fantastical rip off)
⁃ Resizing from all corners
⁃ Full-screen mode for Safari, Mail and iTunes
⁃ Creating spaces with Mission Control
⁃ Power-user centric TextEdit
⁃ Upgraded System-wide Spell Check, Predictive Text and Dictionary (including the new pop-out highlight)
⁃ The gear menu's "encode video" feature (the Automator team snuck this feature in; Sal Soghoian discusses how it got into the OS in his WWDC training video)
⁃ Time Machine is much faster
⁃ Spotlight is much faster, more organized (like it was in Tiger) and now has Quicklook capabilities
⁃ Mail has UPS, USPS and FedEx tracking identifiers and a cool tracking pop-up that doesn't require Safari
⁃ New Preview is power-user centric (the signature feature is awesome; we need Preview.app on iOS)
⁃ New full-screen Photobooth is fun
⁃ Sort by Application Category
⁃ Group selected items into a folder
Things I Don't Like
⁃ Privacy implications (automatically opening previous documents, video files, etc)
⁃ New iCal UI
⁃ New QuickLook UI (why white? Very distracting)
⁃ New Address Book UI (the new UI is awful and why Address Book is not named Contacts, I don't know)
⁃ The restore partition doesn't actually store the Lion installer and requires you to re-download the installer from the App Store while phoning home to Apple your Mac's serial number for verification
⁃ Safari 5.1 is buggy (seems like the Javascript engine just quits working periodically)
⁃ AirDrop requires both computers to be "in AirDrop" in the Finder to function; it won't just prompt the other machine to download the sent file.
⁃ You can't right click and select a file to transfer via AirDrop; You have to drag it into the Finder's AirDrop section
⁃ Making folders with Launchpad is absolutely a step backwards (We have a mouse that has the capability to select multiple items to form a folder, let us use it.)
⁃ File sharing removed "Macintosh HD" from its default share points for some reason (This may be a personal issue with my two Macs)
⁃ Stacks still sucks (and is nothing like the original demo/promise of Stacks, see YouTube)
⁃ Automator contextual actions still don't have the application icon like in the original Snow Leopard demo; they are an unorganized mess
⁃ Front Row is gone (I love how Apple touts the new Mac mini as the Mac for your TV, but doesn't include any type of interface for it; the new Apple TV interface should have been included as the new Front Row for Lion)
⁃ QuickTime X still doesn't support third-party plug-ins
Things I Feel Are Missing or Missed Opportunities
⁃ The "close", "minimize" and "maximize" stop lights still operate differently in different apps
⁃ There's no central media server service feature (like Windows); you still have to have iTunes open to serve media (to Apple TV, iOS devices, other Macs, etc) which is ridiculous
⁃ Lion setup doesn't offer to re-download all your App Store applications on a clean install
⁃ Having both iChat and FaceTime as separate applications is unintuitive
⁃ There's too many Finder views, Finder.app is still extremely complicated
⁃ AirPort Utility still can't perform disk diagnostics (repair permissions, repair disk, etc) on Time Capsule or AirPort Extreme drives; AirDisk drives are still not a supported method for Time Machine