In 12.04, RhythmBox has been ported to GTK3 (i.e. the GNOME3 UI), and is currently missing quite a bit of the old functionality (including visualization); old plugins are not supported either.
A visualization plugin is available in experimental GNOME3, but upgrading to this requires upgrading a lot more than RhythmBox and is not recommended.
For visualization, it's best to wait until 12.10, when the upstream changes from above will probably be included.
For other plugins (lyrics, etc.), AU mod/superuser fossfreedom has been testing and porting them to his PPA so they will work with the stock Rhythmbox; please see this answer for which ones are available and how to install them.