The first thing that i had in mind was speed. I wanted the library to be lightweight and effective. I could live without some features, but performance is really critical. You do not really want your users waiting over client code that is executing on their browsers.
Luckily, there is a tool (in mootools website), that can help us make out which js library is faster. You can find the tool here :
If you run the test (do it, it’s fun), you will notice that mootools, along with Dojo, are the fastest frameworks. So, now you now. Choose wisely