Recs.
Updated
Specs
Pros
Pro Customizable with easy-to-install extensions
With the right plugin enabled in your browser (comes with Firefox by default) you can browse and install with two clicks the many Shell Extensions available from here. These are listed automatically based upon the version of Gnome Shell you currently have installed.
Pro Easy theming
Changing the look (and feel) of Gnome Shell is easy, shell theme, icon, windows and graphical elements (gtk). individually for each user. Mostly it's installing some packages or unpacking some archive to a themes folder and using selecting the new theme in e.g. gnome-tweak-tool. There a lot of really good theme on deviantart.
Pro Fallback mode offers a classic look
For people looking for older, more classic looks, Fallback mode offers just that. Ubuntu users can have this option by installing a package called "GNOME-Session-Fallback." In the future to be released GNOME 3.8, the Fallback Mode will not be included, so this is really not a long-term solution.