Ease of use
Great documentation and tutorials
The JQuery website has a comprehensive documentation and tutorials to get even an absolute beginner in programming to get the ball rolling with this library.
Strong opensource community. (Several jQuery plugins available)
JQuery, while relatively new, has a following that religiously devote their time to develop and enhance the functionality of JQuery. Thus there are hundreds of prewritten plugins available for download to instantly speed up your development process. Another advantage behind this is the efficiency and security of the script.
JQuery lets you develop Ajax templates with ease, Ajax enables a sleeker interface where actions can be performed on pages without requiring the entire page to be reloaded. A pretty but simple example of this can be seen above the fold on