That last part about the ruby class staff was more directed at the original poster’s recommendations, not you, sorry! The hr line isn’t very visible. The whole point of that part was due to retaining the former syntax too. 
And yeah, I guess having some way to add IDs/classes/attributes to some types of elements is just really useful. There are probably other ways but it makes the most sense for the content writer to specify the info if they desire. It’s also much easier than separately maintaining the classifications, as with your cms drag drop interface example.
And if the writer doesn’t want/need to, when it’s not semantically relevant, then they need not worry about it or be affected by it! I wouldn’t say it’s layout specific though since that implies it’s only related to presentation, which it shouldn’t really be.
Not sure if it’s even that advanced either, as this is already a widely used feature with the headings (for example in markdown extra/pandoc’s markdown), but I guess that’s just my opinion of it.
Well john said that he’d like to focus on just the markdown as a method to produce html for now at least. I’ll try to edit in a link to where he says it when/if I find it (assuming it’s not a false memory).
But I certainly know that other output formats for markdown exist. I literally have hundreds of pdfs produced from markdown documents (nearing one thousand soon). Nevertheless, I understand that the primary, default format is html, the current spec itself always refers to html output, so I see no reason for that to hinder having this proposal in the spec. If another output format doesn’t need such information, then it can be easily removed/ignored.