Thanks, Reid. "What is ‘content’?" is a key question to consider as WordPress revamps the editor and customizer, stepping back from the technical issues of TinyMCE, shortcodes, etc., (remember Post Formats?)... The WordPress WYSIWYG experience has been disjointed between the dashboard editor, the customizer, and front-end editing. Now, we are considering a new vision for that experience, but it's not so clear where it will go and how it might be accomplished.

In Drupal the answer to your question is obvious: Node. And what's a node? That depends upon the Content Type. Nodes are as atomic as one can get, though it's rather abstract semantically, compared to how Brad Frost designates HTML elements as the atoms of web content. With Drupal the followup question is how do we construct content at the next levels for useable blocks? There are many answers, and the pathways become complex design and development projects.

For WordPress, blocks seem to be the current answer for something less than a post/page, but there's no consensus yet on what constitutes a block. Blocks are often constructed as a shortcode with page builders, but without clear semantics for that as it's buried in the actual longcode behind them. The current core-editor discussion explores different directions: whether blocks might be text or paragraphs, plain or formatted, or even if they are HTML-based. Matias Ventura commented in one discussion, "There are tradeoffs depending on where we draw the line regarding what is the minimal unit that we consider to be a block."

For a lot of end users content is both the source material they start with—words, images, audio, video, products—and the output on the web that visitors experience. It's simply their content, while WordPress and the web are just communication mediums that moves content out there. No wonder commercial site builders like Squarespace are popular, as they don't get in the way, even if they box in the end experience.

AX ≠ UX. That's something Rick Yagodich explains in his book Author Experience. Web design has focused on complex and beautiful UX, but the content construction process is secondary, and as Karen McGrane summed it up, "CMS is the enterprise software that UX forgot." (Content Strategy for Mobile, A Book Apart, 2012)

You're right that "to push WordPress’ content creation tools forward" we need to focus on defining the elements, their relationships, and how they work with a new editor along with themes, plugins, and other tools. And as we continue to democratize web publishing, let's keep the barriers to entry low for authors, editors, content managers, and all the other content creator use cases.