It was the first library that worked consistently well across the board, provided an easy-to-use CSS-selector based experience to query and manipulate the DOM, and had very solid documentation.
I feel like jQuery is/was a mainstay of the web and although we've seen it lose popularity over the years it's still one of the biggest game changers in my web development tooling.
He also is a key player at Khan Academy* which is one of the best online learning resources to date.
All-in-all I think the guy is an excellent example of an entrepreneurial engineer and I would fanboy so hard if I ever met him.
* Not founder of Khan Academy (doh!)