Paul Thurrott has a pretty good run-down on what's new in P3 here. I have to say that I'm extremely impressed by it so far. It's got a nice interface, only a tweak from P2 actually, but subtle improvements exist throughout. The performance is better and there are new features for both managing libraries and photo images themselves.
I previously used Flickr and a few other online file storage sites, but Flickr started laying heavy fees and upload limits and I just don't like the Flickr interface at all. To me it's confusing and awkward. Picasa follows the standard Google UI approach to being simple and clean, and easy to understand. But without sacrificing features or function. Just my humble opinion.