The only change is that the Play mode button is gone, and the Pages panel was added.
Play mode was redundant as it only highlighted that you are currently using the website. That is, none of the tools are currently enabled. We got rid of that.
The Pages panel is a menu tree for your site / app. You can use it for both navigation and manipulating the pages.
Finnally, most of the changes went into the 2nd tab, which was called Layers panel or File panel. Now it is called the Object editor. Pretty much the entire IDE went into it. Let's start with that because that's the most interesting bit.