Three top features of jQuery: the multitude of selectors, manipulating attributes, and tacking Ajax/JSON requests. Choses jQuery over Prototype
LowPro is a scripting extension to Prototype written by Dan Webb to make unobtrusive DOM Scripting much easier. Included are an onReady event, adding behaviour using CSS selectors, fixing the this reference to always refer to the element within events, and a mechanism for triggering events.
As well as supporting CSS based selectors, jQuery also supports XPath expressions. This page lists a plethora of code snippets demonstrating jQuery's XPath matching capabilities.
A list of the CSS Selectors supported by jQuery. It includes the regular gamut of CSS selectors as well as attribute selectors, pseudo-selectors and child selectors.
A useful overview / introduction of the jQuery library, covering using selectors and XPath expressions to target specific elements in a document, simple Ajax requests, the basic animations available, restructuring a document, creating jQuery plugins.