If it looks like normal text but get rendered to a different text it is a negative value proposition.
I would say this would still fall under the realm of readability and of being lightweight (as in a markdown text looks like someone that did not knew markdown could have written)
I think this isn't exactly fair, in that it seems to mislocate the problem——Markdown itself is so underspecified (I think Gruber never really expected it to take off the way it did) that the meaning of 'Markdown' can vary wildly from place to place, as a result of which there's no simple, universal test suite, and it's hard to tell on what sort of edge (or not-so-edge) cases your homerolled parser will trip until you actually deploy it (at which point people have come to rely so much on its quirks that it's difficult to fix it).
The main value proposition was that the raw markdown looks like normal text. If you just want a simple mental parsing model, you use html or bbcode.