Most notably, Pownce and Digg have been integrated as people services. If you're not familiar with the self-styled "social browser," this means that you can perform all Digg- and Pownce-related chores--sorry, that should be "tasks"--from within the browser's social-networking features. Support for AOL Webmail has also been added, letting you check that account as easily as your Gmail account.
There's still a long list of known bugs that Flock has documented as in need of a fix.
Since Flock is a fork of the Firefox code that's been around for about a year, it will be interesting to see if future Flock updates attempt to integrate any Firefox 3 improvements or if they're going to pick their own migration pattern.