Blogging and Clipping
Flock supports all of the main blogging platforms so that you can blog straight from your browser at the page you want to blog on. Setting up a blog is as simple as going to your blog editor page and logging in. Flock will tell you that it can set up your blog for blogging from any page you've loaded into your browser, and if you say "yes", it prompts for login information and you're all set. The cool part is that if you have more than one blog, it will store and set up all of your blogs to make it easy to post. All you do is select your blog before posting.
Some other nice features: Saving blog posts directly to your disk in case they get lost in the publishing stream, easy toggling between the WYSIWYG editor and HTML source, and spell check.
Clipping
If you've ever had to cut and paste something from one page to another you'll appreciate the Clipping feature. Just select the text, photos or other content you want to copy, and either right-click or click the little clipboard icon on the lower left, and your selection will be saved by Flock for you to use whenever you need it.
All in all, the developers of Flock have really taken the time to listen to what today's Internet users want in a browser. Much of the functionality of Flock can be added into Firefox with Greasemonkey scripts or extensions, but the cool thing about Flock is that they've made it easy for people who are not geeks and don't want to hunt down those extensions.
Flock gets five stars from me for putting the users first. (UPDATE: 6/28 — But, they need to fix the issues with the feeds in order for me to fully endorse and recommend it, which I'd like to do)