http://www.thinkvitamin.com/features/javascript/15-things-you-can-do-with-yahoo-ui

The YUI utilities gives developers the expressive freedom to do what they want. And although comparing libraries is generally silly, the best illustration I can give is the Script.aculo.us Effects library vs the YUI Animation utility. Script.aculo.us offers you some pretty sweet looking effects like Fade, SlideUp, BlindDown, Shake, etc.. You can even add Parallel effects (way to go Thomas). On the flip side, this animation util doesn’t aim to give you these canned effects to package up with your web app, but rather it lets you make up your own effects by giving you the finely crafted tools (like a utility knife) to make what you want.