Jeff writes:
“I’ll also note that 95% of the above commenters have also put carriage returns between their paragraphs. :)”
While this is true, it doesn’t solve the problem. Many of the e-mails I get don’t use two returns between paragraphs, so I’m guessing there’s a number of people who don’t write that way, even though most of the people commenting here do. Pointing out that 95% of the people don’t have that problem doesn’t solve the problem.
Those wo do have the problem have no obvious way of figuring out why Markdown isn’t working for them. The best solution would be to fix the problem, rather than trying to fix the people.
One way of fixing it would be to automatically add a second return character via JavaScript when appropriate, while the user is typing. Another way to fix it would be to change Markdown itself. Markdown ignores single returns because that allows people to format ASCII text columns with fixed widths without breaking the resulting HTML. I think in Stack Overflow’s case, that feature is not necessary.