The new library significantly reduces the complexity of most popular methods in jQuery. The most dramatic changes were in remove, html, and empty methods. It also adds several new Ajax methods. Another improvement is that a data cache is no longer created if it is not needed, which increases performance and avoids polluting the DOM.
An overhaul to queuing improves the experience of working with queues other than the default. These include a .delay(), Queue next, and .clearQueue() methods.
On the networking side, Google is now hosting a copy of the jQuery code on their fast edge cache servers, in order to speed the performance of jQuery apps.
Full jQuery 14 release notes - Jquery 14.com
Notes on jQuery breaking changes - Jquery 14.com