Essentially the same API, but 2.0 will drop "OldIE" support, freeing it up enormously.
My site still has many IE7/8 visitors but I want to use jQuery 2.0. Can I do that?
If your web site needs oldIE support, and we expect most sites will need it for at least another year or two, you can use IE conditional comments to include version 1.9 only when visitors are using oldIE:
<!--[if lt IE 9]> <script src="jquery-1.9.0.js"></script> <![endif]--> <!--[if gte IE 9]><!--> <script src="jquery-2.0.0.js"><</script> <!--<![endif]-->
Why not make an "oldIE plugin" for jQuery 2.0?
Special dispensations for oldIE are sprinkled throughout jQuery. Refactoring the code to provide enough hooks so that oldIE support could be added as a plugin would complicate the code for the modern browser case. Plus, developers supporting oldIE and using a public CDN would then need to include two files (jQuery 2.0 and the oldIE plugin) instead of one.
Continued development and introduction of libraries. Plenty of Addy Osmani resources help to make sense of the mess.
Fundamental question: What's more important, keeping HTTP requests down or having a site immediately responsive?
Movement on the W3 spec front, no winners yet. ES-Harmony path proposed.
The community is espousing linting more and more, made possible by better tools and better IDE and Text Editor plugins/scripts.
Plenty happening in this space.
Covered elsewhere in better detail.