There's 3 types of content:
- Arbitrary written content
This is where markdown shines and IMO completely solves content creation.
- Arbitrary written content with mixed in assets
Nothing works here. Ever try floating some text to the right of an image when you don't have access to CSS or the raw html because we have to assume the content creator has no technical knowledge.
Even wordpress fails horribly at this but it's such a common thing you want to do.
- Structured content that has clear properties
This is pretty easy to design a nice UI for because it's no longer "anything". For example a todo item or a photo that belongs to a gallery. All you have to do in this case is create a form field for each property. It's something a non-technical person could do.