Not sure if this is really within the scope of this standard to change, but this has irked me and several others: * and _ both act exactly the same when producing italics or bold text. This makes markdown a bit more confusing to learn, and to avoid making a mess, the author has to pick one scheme and stick to it.
One solution would be to scrap one of them altogether; using * or _ exclusively. For example, *i* could be the only way to create italics, and **b** the only way to create bold text. Another solution would be to let *b* be bold and _i_ be italics.
As I said, I’m not sure whether it’s within the scope of this standardization effort, but in my opinion, it is a real issue with the markdown standard, and has to be changed at some point. This will break some backwards compatibility, so if it should be changed at all, it has to be changed as soon as possible.