The Library Module
Viewing Photo In Loupe
Double clicking on any photo in the grid will bring the photo up in Loupe, where you can view the photo in a larger size.
Compare Feature
In a photo shoot, a photographer often take multiple shots of a single scene and will pick out the better ones at home or at the studio. Lightroom comes with a Compare feature that allows the user to compare two or more photos at once.
Although this is a useful feature, it is still unrefined. All it currently does is display these photos in the center panel in an orderly manner. You cannot, for example, compare two photos side-by-side at 100% zoom. Neither does it allow the user to rearrange the photos.
I think it's critical for a photographer to be allowed to re-arrange the photos in a flexible manner, whether he wants them to be arranged from top to bottom, left to right or overlapping each other. This is one area that deserves more effort from Adobe.