jQuery is now modular. Based on the build tool grunt, it is now possible to pick and deliver only required modules of jQuery.
Sizzle, jQuery’s selector engine, has been rewritten and is a lot faster now. It shields developers from bugs in the HTML5 querySelectorAll implementations in different browsers.
jQuery 1.8 automatically prefixes CSS3 attributes assigned via .css() or .animate(), so developers can escape vendor prefix hell.
Cross-Site-Scripting (XSS) prevention has been introduced in the form of a new $.parseHTML method.
Besides these new features, more than 160 bugs have been fixed, performance has been improved and code size reduced.
See the official blog post and change log for further details.