First of all - it loads much faster. It supports most mainstream RAW camera formats. It displays photos in folders as thumbnails. It has full screen previews. All of the normal stuff for viewing photos.
FastStone also has pretty good image editing features. It is not Photoshop but it does have a lot of editing features. And it is FREE.
What makes FastStone really neat are the extra features listed below.
1) Batch rename
2) Batch resize
3) Batch save in different format
4) Batch rotate
5) Batch Crop
6) Batch Change Color Depth
7) Batch Change DPI
8) Batch Add Text
9) Batch Add Border
10)Batch Change Color Depth
11) Batch Adjust Colors
12) Batch Change Canvas Size
13) Batch Add Watermark
And what makes FastStone really GREAT is any or all of these features can be combined on a group of photos.
FastStone also has a slideshow function. It will also print contact sheets from selected images. It will generate multipage files from a group of images. It will break up multipage files into single images. It will make an "image strip". This is a series of individual images combine into a "strip" of images. The images can be resized at the same time. This is cool.
FastStone also has a compare funtion. You select multiple images and display them all at the same time. This is good for selecting the best out of similar shots.
The list of functionality goes on. These features are my favorite.
You owe it to yourself to go and checkout the latest version. Someday you'll need it.