All it can do is bold, italics, links, lists, and blockquotes. That's pretty basic. It doesn't support headings or any of the more extended stuff that even reddit does.
True, but I think that it would be better if it all was implemented on the server side. Otherwise, people who don't use a script with extended syntax handling would just see a garbled mess.
I could try and contribute code to the Hubski markdown but it's not really something about which I should be making any decisions. Also, and I don't mean to slag forwardslash by saying it, 'extensive' isn't the same as 'having extensive front-end functionality' when talking about code. That repository is pretty big and does a lot of crucial things. I simply don't have the time to go through it and implement those things you want.