Pleased to hear you are trying it out! The release notes published on Our outlines a bit of what you should expect, so definitely recommend having a read there:
https://our.umbraco.com/download/releases/1400
Specifically, media and members are not done yet. It won't be feature complete before the RC in terms of UI features, but what we promise is a stable API for extensions, and if we do need to break it, we'll document it.
The next beta should include media creation in the UI as well.