The Good Doctor writes:
Fundamentally, Markdown was tolerant and inclusive. At the risk of some ambiguity, it let you write pretty much the way you’d write a nicely formatted plain text email and turned it into HTML for you. It was both easy to write and easy to read in source form. The readability of Markdown was key. If a normal person could read your Markdown source and understand its structure, chances are Markdown.pl could, too. Gruber's Markdown, as outlined here, is the canonical version for me. Like Drang, it's never lead me astray and is the one I reference. Furthermore, I'm in complete agreement with Drang that the best parts of Markdown are (a) its cleanliness and (b) you're able to drop in raw HTML code when needed. The simplicity and versatility of the syntax is really wonderful. Related: my piece for TidBITS from last year on Markdown and accessibility.