This is sort of my problem with the whole genre of "light" markup formats.... ultimately they are still a bunch of arbitary conventions to remember, and at that point you might as well just learn/write HTML. YMMV.
Strictly speaking, that isn't the problem being solved by Markdown, either.
Remember that Daring Fireball does not have comments, so this isn't a concern. Markdown was something originally created to solve an authorial problem, not something for forum creators to use for comments.
It's flexible enough to do that, but it isn't the purpose of Markdown.
Why not? You have to protect against malicious HTML injection whether or not the user is using markdown, plain text or html, so why not let them use a carefully restricted subset of html?
While this is not wrong, markdown involves much less typing than HTML. Also, should one need to use HTML, markdown does allow one to switch between HTML and markdown.
They're arguably less arbitrary than HTML; most of them are based in things that people have done/tend to do with plain text when they're trying to create a formatted document. That yields two benefits: the source document is inherently formatted -- transparently so, since even people who don't know what Markdown or the like is will be able to read it -- and creates something of a visual mnemonic for the formatting convention.
Not saying any one implementation is perfect, just that they have these nice properties.
