To me the biggest feature of Adobe Photoshop Lightroom 3 (latest version is 3.2) is its nondestructive adjustment ability, that is when you edit your photos in Lightroom, you do not alter the original files. It is more common to video editing software where you can preview of the movie under construction, and once you finish your work you just render into desired format. The similar happens to your pictures in Lightroom, and if you like me mostly use jpg format, which is not suitable for repetitive savings, that feature allows to show you a preview of any editing changes. Your edits are stored in Lightroom as a set of instructions that are applied to your photo only in memory.
And do not forget about the cool and essential key commands (keyboard shortcuts) in Adobe Photoshop Lightroom:
Tab — hide and show side panels
Shift+Tab — hide and show all panels
F — Cycle full screen mode
L — Dim the lights in circle
` — Flag the selected photo(s)