One thing I like is the ability the combined ability to set permissions on photos (for friends or family) and the ability to create invite-only groups with a shared gallery. Along with the excellent notes, comments and tagging facilities means it's possible for friends to collaborate on recording an event in quite a comprehensive manner. There's been one deal-breaker in all of this, until now that is. It's now possible to easily manage batches of photos, and add them to groups or sort them into galleries (photo sets), using a new Flash tool called Organizr. It's very swish for a web-app, and I assume it will only get better.
Today also marks the release of Flickr's full Services API which I'm going to take a good look at soon. I don't think I'll be able to resist knocking up something for Flickr using my other current distraction, Processing.