I know Gnome 2 is history but the Gnome Classic session looks (to me) exactly like Gnome 2 i.e. as per
Are there any differences between the two desktop environments?
Well there is the obvious answer, that the libraries are very different and often incompatible with each other, but since GNOME 3 uses the newer libraries that is preferable. At this point considering the work that has gone into Gnome Classic for 12.04, I would say no, that there isn't really much of a difference at this point. Maybe a few taskbar applets that haven't been ported yet, but aside from that there isn't really much point in using Gnome 2 anymore.
There are significantly less panel applets which work with the panel in the GNOME 3.x fallback mode. Some things have been hidden away or removed as well, to try and keep the fallback GNOME session more similar to the new GNOME Shell.
As a long term *Nix user that was quite happy with the Gnome 2 desktop (but not it's bugs), I'm pleasantly surprised at the current status of Gnome 3 'Classic'. The difference to me between the two is that the newer version is much less customizable but more stable (my Applets no longer disappear, for instance) and consider that if enough resources are given to the 'new - Classic' (I know), it has the potential to be as good or better than Gnome 2 (or MATE)