I’ve been using iPhoto’11 a lot more in recent months after being a heavy iPhoto’09 user. Three observations:
1) they changed a lot of UI to make it appear slicker. Generally, it’s a lot less intuitive though. For example, to edit the date of a photo you need to go to a menu option instead of editing it right in the info pane.
2) face recognition got better (no surprise). Happily they added a UI for wading through all unnamed faces, but it’s hard to find and won’t let you look at the whole photo for context
3) behind the scenes, the SQLite database that holds metadata is completely different. All the same data is there, but the joins all need to be rewritten from scratch. I suspect the new schema comes from Aperture, but it might just be a CoreData rewrite.