According to Ian Hickson, who maintains the Acidtests website, there have been several revisions, including SVG Fonts, a feature that was criticized by Mozilla. Hickson said that changes also affected SMIL animations, Xlink as well as DOM Range. Overall, it appears that the developers cleaned up the test and removed antiquated and unusual web technologies. I can't help but feel that much of the changes are a late reply to criticism that has persisted over the past two years.
Acid3 itself is more than three years old and could use an update, possibly in the HTML5 range where the support of confirmed and planned technologies is pretty much a mess. Since all major browsers now achieve a perfect score in Acid3, there may be little reason to use Acid3 in comparison tests at this time.