v0.3.0
Editor UI Refresh
Version 0.3.0 contains completely rewritten Deck, Show, and Slide editors. The new look is based on missing.css to better integrate with the rest of OpenShow's UI, and is far more intuitive to navigate.
Here's a comparison, with 0.2.1 on the left and (new, shiny) 0.3 on the right:

Those screenshots both show the Show editor, but the Deck editor has received the same treatment. All functions are far more obvious. In particular, AOML editing is now far easier.
Media Handling Changes
Caution
Breaking change alert!
Media Objects have now completely replaced the old SlideElement.video system. That attribute has been deleted. If you're using it, please manually replace all instances with Media Objects before upgrading to this version.
With that disclaimer out of the way, let's get to the good stuff! This new approach means that media (audio and video) is automatically transcoded to a format compatible with both Firefox and Chromium (important for displays used in the OBS Browser Source).
Media Objects also support entering a URL for a Vimeo livestream, which will be displayed full screen and set to autoplay when displayed.
Image Handling Changes
Images now also have a dedicated model, which allows for greater flexibility in data storage. In the future, the Image model can also be extended to optimize the size of images for quick slide loading, but this is not yet implemented.
AOML Spec Updates
OpenShow contains an integrated markup language called AOML. In version 0.3.0, the initial spec has been formalized and written down. This makes it possible to manually create slides very quickly in a standard text editor.
Finishing Up
Above is a list of the major projects which have seen completion or substantial progress in this version. If you're really interested in absolutely everything that's happened, see #38 for a full commit log of the changes from v0.2.1 > v0.3.0.