This is going to be a huge win for WordPress users. Even with editor styles, it’s still a bit of a surprise how your content will eventually look. I tend to write directly in the WordPress editor, and I find myself constantly switching back and forth between editor and preview to see how my content looks.
This would eliminate that step, and be a significant improvement over the current writing/preview experience.
This ongoing push to standardize front-end editing may also have the unintended side effect of adversely affecting commercial WordPress product businesses (like Aesop Editus) that deal in similar areas.
Overall, I think it’s a good thing, and will help WordPress better compete with the likes of Wix, SquareSpace, and Medium. What do you think?