First most notable improvement, events. In iPhoto 06, photos are first organized by "rolls," and then you have to create albums manually on the sidebar. In iPhoto 08, organization is much easier since iPhoto will automatically create an "album" for you when you upload your pictures. They're called Events. The arrangement of "events" is much easier to users then "rolls."
Unfortunately, I found the "quick view" feature somewhat annoying. When I double-click a picture, instead of automatically go into editing mode, iPhoto only enlarge the picture. To edit it, I have to click on the edit icon. Annoying extra step. The picture adjustment features is better than 06 though, having clearly labeled sliders like shadows and highlights, and finally a dropper icon to choose a neutral color instead of command-clicking.
Although I still think Picasa to be the better software, I find it much easier to use iPhoto 08 compared to 06 thanks to "Events," and haven't touched Picasa ever since.