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.
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.