I tried to think of a good lion pun of the title, but I couldn't. I'm not doing a full review, since there are literally hundreds around the internet. Instead of all the really exciting, huge new features that Apple wouldn't shut up about, I'm going to highlight some of the smaller additions to OS X that have improved in Lion.
External displays and laptops
It used to be that to use an external display with the laptop closed, you had to attach the cables, including the power cable (no running off battery power), then shut the lid on the laptop. The computer would then go to sleep, and to use it on the external display, you had to use an external mouse or keyboard to wake it up. With Lion, you no longer have to wait for it to go to sleep and wake back up again. When you shut the lid, the transition to the external display is automatic. You still have to keep the power cable plugged in, though. I don't know why Apple thinks using a display when on battery power is such a bad idea.
iChat
In Lion's version of iChat (version 6.0) everything seems much cleaner. The shiny chat bubbles have been replaced with pastel colours, and if you have multiple accounts, the contact lists are merged into one window. Duplicate contacts from different accounts are even hidden so each person is only in the list once. (You can choose which account you want to send the messages to, but I don't think it really matters too much.)

System Preferences
The best new feature of System Preferences is the ability to hide preference panes from the main screen. If you click View → Customise… you can uncheck the ones you don't want to see. I use this to hide panes I never use, like Parental Controls.

About This Mac
The information you're presented with if you click About This Mac in the menu is much nicer, and is also more in-depth. If you choose More Information, you'll be shown an overview of your computer. It even tells you which type of computer it is (by which I mean mine says MacBook Pro, 15 inch, early 2011).

If you then click the Storage tab at the top, you'll see an iTunes-esque breakdown of the drives you have mounted, showing you how the space on your drive is taken up. It's much more informative than About This Mac used to be.

Link preview
If you receive an email or IM containing a link, you can hover over that link and a small triangle icon will appear. Click that, and you'll be shown a preview of where the link goes in Quick Look.
Double-tap in Safari
In Safari, you can now double-tap your trackpad or Magic Mouse and you'll be zoomed in, exactly the same as on iOS.

New downloads list in Safari
Before Lion, Safari showed you downloads in progress using a separate window – one of the most annoying features ever. In Lion, it's integrated into the toolbar, and only visible when you're actually downloading something. You can click the toolbar icon to open a popover to get more information.

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