Mostly KDE itegration and support for a really broad array of formats. Most people probably don't use it as such, but it's more of a general document viewer. (For example, you can actually open Microsoft .ppt files with it.)

It's a great pdf viewer, and has plenty of features, but so do most of the other well-known ones.

I love it, but I'd probably use something else if I wasn't using KDE as my desktop environment. It's lightweight if you're already loading all of the KDE libraries, but rather heavyweight otherwise.