Some of the features I like:
Web Clipboard: allows you to drop blocks of text or photos for easy inclusion into your email or to save for later use. The clipboard opens as a task pane on your left and retains your clippingsPeople Sidebar: Logs in to your stored social network accounts and gives you up to date information about their status, alert you when they uploaded new media and gives you social network actions directly from within the sidebar
Flock has all the other features you expect from a modern browser: remembering username/passwords (works better than Firefox's own and IE); When you close and reopen - all prior browser tabs are reloaded, etc
There are some slight bugs in the beta version I tested (v2.0b2), specifically that sometimes the browser back buttons don't work , but I use the mouse back button or the Alt-LeftKey shortcut to circumvent that. I assume it will be fixed in the final release.