You could even simplify and say that Jekyll is a build script

It depends on what your needs are. If you need a central web interface with multi-stage workflows, multiple authors, image management, future publication, etc, then Jekyll isn't a CMS. But Jekyll focuses on content, handles markdown, reads metadata, and uses templates. For some people, that might be plenty. A pretty web interface isn't a must-have requirement.

> A pretty web interface isn't a must-have requirement.

According to most definitions of "CMS", it is. With static website generators, editing, versioning, publishing, etc is done externally. They aren't systems which encompass all of this. They just punch some content into your templates and write the result to disk. That's all they do.

Are there any CMS that have a "pretty web interface" ?

From what I've seen of Drupal, the interface exposed to content creators is awful. It works (mostly), but it ain't pretty.

I like Plone a lot - it mixes the admin with the site itself.

CQ5 looks better (but isn't).

Oh, you.