Lots of people that have been using YUI2 will appreciate the new YUI3 modular concept. While using YUI2 if you wanted to use a basic version of a YUI widget/component you would have to include the all of that widget library. Now that has that behavior has changed and most of the widgets/components/modules have been divided in several smaller chunks that make sense. For example, YUI3 IO has io-base, io-xhr, io-form, io-queue etc. You could load any one of them depending you needs. No need to load all of IO.

YUI3 has chaining similar the jquery (not as clean, I think) which makes is less verbose and pleasant to write JS.