"Item #3, the conversion of returns to linebreaks, is somewhat more debatable. I'm on the fence on that one, but I do believe it's significant enough to warrant an explicit choice either way." Really? Has anyone ever typed stuff, hit enter once, then typed more stuff, ever not intended a line break? "Oh no, I kept hitting Enter between words instead of using the space-bar! If only pressing Enter was interpreted as a…
Edit: to answer the question of the comment below this, poems need to be handled specially in Markdown as in HTML. It's an issue more generally, and I think that the standard solution is
for the poem as a whole and
(or depending on your markup flavor preference) for the lines. It's a known thing: http://blog.signified.com.au/a-poem-element-for-html5/