When looking at a adding AJAX you have a couple decisions you'll want to make up front. One is what tools your going to use. In the webthumb case thats pretty easy. Webthumb is a simple PHP app and doesn't use a framework, so I need a nice general PHP/AJAX framework that is easy to use, HTML_AJAX fits that need.
After picking my tools I need to decide what my goals are. My main focus will be to improve usability, but I also want to use AJAX to make the site seem a bit flashier, so its a bit of a technology demo too.
He uses his own current project, Webthumb as an illustration of his points including looks at remoting, a RequestStatus class, exporting a class with the HTML_AJAX package, and finally, making the Ajax calls.