A Mint desktop looks and behaves like a Mint desktop and this one feels both like Gnome 3 and the traditional Linux Mint desktops that preceded it. You can launch applications from the top left, easily switch between applications and workspaces using the window list or keyboard shortcuts, keep an eye on your notifications at the top and access Gnome 3 features like activitiesfrom the top-left corner.
The fallback mode is present in Gnome 3 because it requires hardware acceleration, and Gnome 3 is not compatible with all hardware yet. In fallback mode, you will still run Gnome 3, just without the looks. It is like a stripped-down version of Gnome 3, designed to run on generic hardware of all type.
Gnome 2.32 is there too!
Mint 12 is coming with Gnome 3 all right, but they have plans to include Gnome 2 as well. A big challenge in this was the inclusion of Gnome 2.32, which is incompatible with Gnome 3. Open Source software brings along a flexibility and this gave the Mint team MATE which is a fork of Gnome 2.32. The good news is, MATE is compatible with Gnome 3 and there is a chance that it will be included in the Mint 12 Live CD.