Markdown is a plain text markup language--like WikiText, Textile, or
syntax is heavily influenced by
popular conventions for writing plain text email. For instance, surrounding
text with underbars (_) or asterisks (*) like _this_ denotes emphasis and
turns into this. Footnotes like this  turn into links like this,
and blockquotes like this:
> Jass, Hugh wrote: > bla bla bla. bla bla bla > bla bla
turn into this:
Jass, Hugh wrote:
bla bla bla. bla bla bla
It's also really easy to inline HTML (or XML) when Markdown doesn't let you express what you need.
: WhyNotMarkdown.txt "This page in markdown"
What's The Problem?
Markdown is too easy and natural. You get used to it and then you have a bad attitude towards "real" markup like XHTML Strict. Try playing with Markdown for a weekend and then go to work and jump into DocBook. You could lose your job due to the demotivating effect this God-forsaken syntax has on your ability to deal with the real world. Please don't make the same mistake I made and integrate Markdown into your publishing system. I wouldn't wish this on anyone.