I think the coolest and maybe most misunderstood part of Markdown is that it is designed to make README files into reasonable HTML. In the process of designing something that made for human readable text files that can be easily translated into HTML it became one of the biggest text input standards on the internet.
Like JSON, on its own, Markdown is a nice little tool, but what people have picked up and done with Markdown is really inspiring.
This is wrong. Gruber designed Markdown for writing blog entries, so Wordpress is using it for the original purpose. README files are just one of these additional purposes you allude to in your last sentence.
Also, Markdown is not a standard (yet), and the ecosystem suffers greatly for it.
If we're chasing the canonical rabbit, Markdown wasn't designed for any specific purpose. It was designed upon a core principle: that Markdown's syntax should be as readable as possible. That is, Markdown should be readable before it has been converted to any specific machine-readable format.
Straight from the author:
> The overriding design goal for Markdown’s formatting syntax is to make it as readable as possible. The idea is that a Markdown-formatted document should be publishable as-is, as plain text, without looking like it’s been marked up with tags or formatting instructions. While Markdown’s syntax has been influenced by several existing text-to-HTML filters, the single biggest source of inspiration for Markdown’s syntax is the format of plain text email.
Like JSON, on its own, Markdown is a nice little tool, but what people have picked up and done with Markdown is really inspiring.