Gtk+ 3.2 introduces many smaller features which users will benefit from straight away. CSS themeing support has been improved, GtkFileChooser has been overhauled, the GtkFontSelection has been replaced by a new set of GtkFontChooser widgets, and several new widgets have been introduced as well.
The HTML5 support in Gtk+ 3.2 is still experimental. It allows you to run Gtk+ applications inside HTML5-capable browsers - and we're not just talking a calculator or a notepad application, since GIMP runs just fine in Firefox 4.0 as well, as the video below demonstrates.
The other big new thing - also experimental since, you know, Wayland itself is highly experimental - is support for the Wayland display server. This is all work done to make sure Gtk+ is ready for the future, since all indications point towards a Wayland future (instead of the X.org we know today). Qt is also working on Wayland support, so it would seem the two major open source toolkits are getting ready for the future.
You can download the code, or wait for your distribution to include it in its repository.