Thank god, I was thinking it was some stylistic choice to have to read all those links inline in unrendered markdown.

Why won't trimmed URL simply redirect to a proper one? Or throw 404 even? This seems like a webserver misconfiguration of sorts, kind of like being able to access raw PHP pages, instead of rendered ones.

I'm guessing write.as is supposed to support multiple languages, and only renders the markdown if you have .md as an extension.

Which is the exact opposite of what I expect and want -- the bare filename should render, and the .md extension should get the markdown source.

Agreed. Write.as is open source, you can file a ticket against WriteFreely on github

If the source can be different languages (like md, html, rst, org-mode, wikitext) and the language isn't stored then this choice makes text.