I'd argue that if you are writing text, you'll put the description first and then look up the link. Because the description is often part of the sentence, and the link is not.

In fact you may write the entire thing first then look for whatever can or should be hyperlinked.

Most people wouldn't be able to explain to you what a HTML tag is, much less write one.

And the entire point of Markdown is to get something that is closer to how we write plain text.

I'm more comfortable with the text coming first, followed by the link. The rails `link_to` helper is:

> link_to "Text", "http://example.com"

And Markdown:

> [Text](http://example.com)

It's pretty easy to remember the order after a while.