In reply to Danny Brown.

@Danny: Medium can make such an intuitive editor because they have complete control over their platform: they have a closed ecosystem. They know all of the elements that can be displayed in an article and they can craft a seamless visual editing UI for these elements. I see Medium supports adding images and URL embeds into article content. Images are added via an upload UI, and the UI for URL embeds is just pasting in the URL and hitting enter. The WordPress visual editor largely supports these already. But WordPress is an open ecosystem where you can install thousands of themes and plugins that extend the editor and customize how the content is displayed:

Representing shortcodes in a way that can be edited while displaying it identically to how it will be presented on the frontend is a big challenge here, but one that is being worked on in the Shortcake project.

Consider also plugins that add filters to the_content to modify how it appears when rendered on the frontend. Even in Core, the infamous capital_P_dangit filter. In the editor you can write “Wordpress” but on the frontend you will see it as “WordPress”. Supporting such filters is a big challenge for a seamless inline editor.

So yeah, I would say WordPress's extensibility is the primary reason why there is no seamless inline front-end editor in Core. It is a big challenge to add such an editor while also support infinite variations introduced by themes and plugins.