jQuery has always made feature changes/additions on .x releases. We're saving the 2.0 moniker for removing oldIE because that is a much more significant change than these API cleanups.
At this point the jQuery installed base is so big that any change, however innocuous, is a breaking change for someone. Even fixing bugs, conforming to W3C standards, or making API return values consistent can cause trouble because someone depends on the old way of doing things. We try to be respectful of people's existing work but also want to make forward progress.
Like the intro at the top of the upgrade guide page says, this list seems a lot more scary than it really is. We're just trying to call out every change that we think may cause issues, so people can assess and address them in advance.
The jQuery Migrate plugin is designed to make it easier to find any compatibility problems, and can be included with jQuery versions as old as 1.6. Add the plugin to your current code, run it, and view the console to see if there are warnings.
jQuery 2.0 (early 2013, not long after 1.9): This version will support the same APIs as jQuery 1.9 does, but removes support for IE 6/7/8 oddities such as borked event model, IE7 “attroperties”, HTML5 shims, etc.
Our goal is for 1.9 and 2.0 to be interchangeable as far as the API set they support. When 2.0 comes out, your decision on which version to choose should be as simple as this: If you need IE 6/7/8 support, choose 1.9; otherwise you can use either 1.9 or 2.0.
http://blog.jquery.com/2012/06/28/jquery-core-version-1-9-an...
It would also be nice for them to adopt semantic versioning with 2.0. i.e. move to 2.0.0