Earlier this year Moodle.org added a graphic designer to their team which might be a driver of some aesthetic changes in Moodle core. A first of many improvements will soon be realized on Moodle sites far and wide. For a sneak peek of the new Moodle icon set you can checkout QA.moodle.net which is running 2.4 beta. Moodle 2.4 is expected for release next Monday December 3rd.
A full comparison highlighting the less crowded Moodle look and feel is available at http://docs.moodle.org/dev/2.3_icons_versus_2.4.
A few of the icons: