The Plugin and Site Templates have undergone a massive overhaul.
ConnextCMS now has hooks to allow the development of Sites, similar in nature to WordPress Themes. Sites allow new websites and web apps to develop their code base totally independent of the KeystoneJS or ConnextCMS code base. Separate ConnextCMS core and Site code can be updated without one breaking the other.
Plugins build on top of Sites by adding the ability to create a new left-menu item, Backbone Views, and Backbone Models to add new user interfaces to the ConnextCMS Dashbboard. Like Sites, they allow independent code bases, so you can focus on your plugin, not the ConnextCMS core code. Also like Sites, they also allow you to update ConnextCMS any time without adversely affecting your Plugin.