In the last year though things have changed. All the good stuff about mootools is still good. However jQuery is being adopted by many open source websites and projects and is getting a larger amount of development and documentation work than mootools. They project has risen to the challenge of addressing code efficiency and download size. The jQuery community has shown itself to be open and friendly while the mootools community is starting to feel elitist.
For the record, dojo has split its mammoth library into a few different packages now so the core, which is very feature competitive with jQuery and mootools is also now in the same ball-park for download size. The benefit of dojo is that the mammoth code is still available as plugins providing the widgets and data visualization features, ready to load when needed.
Maybe its time to start re-evaluating the JS library I use.