I think everyone should give one more pass at MooTools. I know that jQuery is very popular right now but MooTools is designed from an object oriented perspective. So for people like us, who are already using Java, MooTools just feels more natural. If you look at the MooTools documentation (
http://mootools.net/docs/core) it shows you how to easily create classes, use inheritance, implement composition, extend with categories et al... Where as jQuery is kind of like a giant collection of scripts that is very useful for people who are more familiar with that style of programming.