In this release JQuery core developers threw out the support for Internet Explorer 6, 7 and 8 (Yay!). Thanks to this, the size of the library has decreased by 12% and now it's smaller and faster. It can be even smaller if you make your own build, disabling some of built-in default modules (there is 12 of them).
Those who need old IE support should remain on the branch 1.x, which will continue supporting old IE versions.
Official post : http://blog.jquery.com/2013/04/18/jquery-2-0-released/