From the mailing list release announcement, "GNOME 3.4 is the second major update of GNOME 3. It builds on the foundations that we have laid with 3.0 and 3.2 and offers a greatly enhanced experience. The exciting new features and improvements in this release include a new virtual machine and remote access application, a completely revamped web browsing user experience, integrated document search, first-class web applications, better graphics tablet support, application menus, and many more."
The GNOME 3.4 release notes cover the changes for this latest release in great detail.
There's also my interesting features of GNOME 3.4 article from earlier this week that goes over some of the most prominent changes for this latest desktop release.
For those wanting to try out GNOME 3.4 in an easy manner, one of the ways to do so while having a great user-experience would be to try out a Fedora 17 "Beefy Miracle" development snapshot. Fedora 17 with GNOME 3.4 as its default desktop will be officially released in early May.