Gnome3 classic mode is mostly just Gnome3 + some extensions. The user interface is only superficially similar to Gnome2. Most things feel crippled compared to what came before it and it loses functionality on every release.
Mate is excellent and truly deserving of the title "modern Gnome2". It's my desktop of choice. Under the hood it's using a lot of Gnome3 tech while the user interface is familiar and usable for desktop users. The problem remains, however, that it is stuck with either a languishing GTK2 or at the mercy of Gnome with GTK3.
I don't have gnome installed anymore, but a few things I remember:
The window list extension doesn't allow you to change the order of windows with drag and drop and I'm not sure if its possible at all. You also have limited options when you right click on them.
The workspace switcher as part of the window list extension or the stand alone extension are now text only menus that require multiple clicks to change desktop.
You cannot drag and drop launchers from the application menu anymore, in fact you can't have launchers on your panel at all anymore.
The system monitor applet is not visible on your panel anymore.
gnome-control-center is extremely limited, you must also learn to use gnome-tweak-tool and dconf-editor.
Also, classic mode just looks ugly and there is probably no way to change it without getting your hands really dirty.
Many of the things listed above used to work in classic mode but the functionality was removed in newer releases. There seems to be a trend of removing functionality from the desktop and moving it all to the activities overview.
This essentially IS LXDE.. That project's developers merged with another team to work on this. LXDE's going to be maintained for a while, but I imagine it won't get as much attention anymore.
