In reply to Johannes.

We're a long way from any sort of finalization on this. And, I'd love to see more about the overall vision from the Gutenberg team. Right now, a lot of this is speculation and experimentation. The following is how I envision the system working based on what we currently know.

The role of themes will be reduced and will cover three primary areas:

Themes will offer unique styles for block output. This will be the primary purpose of a theme -- styling content.
Themes may optionally register custom block styles to offer extras to users.
Themes will set the default output of templates.

The last point, which is what this post covers, is kind of a big question mark right now in terms of how it will work. The idea seems to center on templates simply being a collection of blocks. That's not much different than the original purpose of templates. HTML is essentially blocks. What the block system does is standardize the code so that it can be interpreted by WordPress and also shared between themes.

Where themes really come into play here is they give the user a starting point. This is where the theme designer presents their vision. Many users will simply use the theme and not change anything just like they do today (WordPress currently has a built-in theme editor). Some users will want to change minor things, such as moving the post category to output below the post title instead of after the post. Yet, other users like the general color scheme and block styles of a theme but want to change the overall layout and other major elements. Because theme templates are all built on this standardized block system, end-users can move these parts around without breaking things.

The big change essentially comes down to themes being built in a standardized language (blocks) and end-users having the ability to make changes to templates without editing code.

I'm still on the fence about whether this is a better direction than simply allowing themes to register custom block areas and have a little more control.