The document outlines the significant updates introduced in jQuery 1.5, including a complete rewrite of AJAX methods for improved consistency and performance, the introduction of Deferred objects for better handling of asynchronous operations, and new methods such as $.parseXML(). It also discusses enhancements in the build system, performance improvements in traversal methods, and new features like prefilters and converters to customize AJAX requests. Overall, the update aims to streamline API usage while providing backward compatibility with older methods.