Among the features of the GLX-Dock 3.0 release is a "greatly enhanced" taskbar, a re-written log-out applet, the dock can now be controlled from the keyboard with various shortcuts, a new Twitter applet, Cairo-Dock is now written using GTK3 for better integration with the GNOME desktop, additions to the D-Bus API, improved text drawing, and other enhancements. This new version also allows it to be used as a session manager (Cairo-Dock Session).
Those wanting to learn more about GLX-Dock / Cairo-Dock 3.0 can find out the full details and download links from this forum thread. There's also more details about this 3.0 release on GLX-Dock.org. Embedded below is an official video of GLX-Dock 3.0 in action while running Ubuntu 12.04.