Now don't get me wrong, popular != good. But, it just so happens that jQuery is a really good choice for novices and experts alike. We've chosen it because it has simple syntax that's easy to use and read, well documented examples, and a strong community. Its sheer popularity means you'll have no problem taking jQuery to the next step if you like it and its simple syntax ensures that you'll know enough about what's going on to rewrite some of our examples in pure or using another framework. When I have some more time I'll get a blog post up about what makes a framework a good thing and more about why we chose jQuery even though none of us have used it before extensively.

Personally, I've played with Dojo, Prototype, and mootools and while each of them has had its strengths, each of them has left me wondering why I wasn't using a framework earlier.