Apple's website already has a list of new features for the new iPhoto 5. I only have two complaints:

1. Several websites (e.g. Macfixit) has already stated that people have had slow performance while navigating with larger thumbnails. I can confirm this. Once the thumbnails are made smaller, scrolling through my 2000+ library on my G4 iBook is a breeze.

2. Creating books is rather buggy. I've tried using all of the functions provided with iPhoto 5, and I've only had crashes when I tried to work with photo books, particularly for the "Travel" theme. Also, the "automatically fill-in photos into the Book" option doesn't do a good job of inserting all your photos. As a result, there will be occasionally blank pages within the book when they could've been taken up by some photos.