“One important note before we begin: Many of the changes in WordPress from 1.5 to 2.0 are under the hood. They are things that you’re not going to notice unless you are developer. There are some features that casual users will notice that are significant, but (in my opinion) most of the real change has happened where most people won’t see.”
There is now some AJAX goodness in the admin sections. Including being able to add categories right from the Write Post page, yay! You will be able to browse for images and upload them directly from the write page. There was a bump in version numbers from 1.6 to 2.0, so there must a large number of improvents over the current 1.5.2 release. From the sound of it the code under WordPress is a giant improvement. This is something I probly will never see, but great for the people who create the plugins. (I have no programming skills 🙂 )
The beta is currently out now, but I am gonna wait for an official release.