As they're saying over on the WordPress Development Blog, this upgrade is:
"...a very important update to apply to your sites as soon as possible because it fixes a core security bug in our HTML sanitation library, called KSES. I would rate this release as “critical.”
I realize an update during the holidays is no fun, but this one is worth putting down the eggnog for. In the spirit of the holidays, consider helping your friends as well."
-The Modwest Team