For me really it's been the organisational stuff. That and it seems happy to work with my library stored on my ReadyNAS, something iPhoto really didn't seem to like to do. It also handles RAWs much better, when I started out with iPhoto it was all JPEGs off my little Ixus. As soon as I moved to my Nikon D80, it really started to bog it down.

The image manipulation stuff seems more in depth, but I've not had a proper chance to play with that yet.