The Durandal 2.1.0 Prerelease contains many bugs fixes, some new features and some performance enhancements. There are no breaking changes to APIs and behavior should remain the same as before (minus bugs of course).
A detailed list of the changes in this version can be found here. While we try to avoid breaking changes in minor releases, there are a couple things you'll need to look out for. First, we now support Bootstrap 3.0. If you are using the modal dialog plugin, this may affect you. Fortunately, the new modal dialog plugin has configuration options that will let you fix things up if you are using an older version of Bootstrap. You may also need to make some modifications in
durandal.css. Second, we've updated the example "entrance" transition to use CSS3 animations for better performance. This requires the updated CSS provided with Durandal. Bear in mind, the "entrance" transition is meant only as an example transition. You can/should consider writing your own. If the new version doesn't work for you, you can just drop in the version from 2.0.1 and it will work fine.
Note: The starter kit and samples have been updated with the latest Bootstrap and animations. You can have a look in there to see how it's all working.
Sr. PM Manager at Microsoft leading the APEX Online Experience Platform team. Creator of Aurelia. Opinions are my own.