Why did I leave it so long before I started using this library?
I have seen quite a few libraries in my time, from ActiveX to Zend and across multiple programming languages. In my experience I've preferred those libraries that are modular and highly conforming to object oriented disciplines.
Okay. This is what I love about it:
It's easy to learn
Some clever clogs just cannot help creating libraries that use sophisticated techniques that may be elegant in their solution but are understood by few. In contrast, jQuery is one that is intuitive and easy to use.
Write less code
Essentially that's what all libraries should provide, otherwise you could write the stuff yourself. You can do some really cool stuff with just one line of code.
Reuse of standard notation
jQuery uses the CSS syntax notation standard as arguments in its functions. That is a great idea as it means there is less to learn.
How many times I have had to rewrite huge swathes of code so that it works in bloody IE? This has got to be one of the most compelling reasons to use jQuery.