My biggest gripe with markdown for writing is its removal of single intentional newlines.
If it was like plain text, with a few ascii symbols used for formatting (like *, _, ...), it'd have been more fantastic than now.
BBCode is better imho.
The only two markdown features afaik that work on HN are italics with asterisks and code blocks with 4 spaces.
That's it.
I prefer just plain "<br/>" because it does not mess with my editor configuration, which otherwise I have to constantly tweak between "strip all whitespace at the end of line" and back.