I've been lucky enough to have been involved in the testing of the application for some time. I've also been working with David Schloss at maccreate.com to write tutorials and do videos for the launch.Everyone will love the new Brushes and Presets feature, but will be totally blown away by Places. Even the simple stuff, like metadata management is much better. The import speed is super cool because you don't have to wait for it copy over before working on the images. On top of quick brushes, nearly every adjustment is brushable. I could go on but take a look at this run down on maccreate.com.
Also, check out the Aperture 3 adjustment preset package Sara France and I are selling on maccreate.com.