While I hate to do so, I will “swear” that I heard voices singing the day I first discovered jQuery. I found it to truly be G*d’s gift to UI programming. No more carrying around large bulky libraries that of seemed to have to be tailored to match the markup of each new application. While I do employ the various other JavaScript libraries from time to time, with the ever-growing power of the jQuery library itself and the wealth of code snippets available, I rarely find it necessary.
As for AJAX, it’s my new thing. While it’s nothing difficult, the need for it in brochure sites and sites that are to be optimized for SEO, is very rare. However, my recent dedication to WP plugin development has brought to light a wide array of applications, and I utilize it more and more each day.