OMG no. RST has such a random syntax it's impossible to remember anything. I really hate it (have to use it at work, its processor is slow as hell, just a horrible experience).
Agreed, the RST syntax is truly awful. At least Markdown is easy to understand and work with.
Org-markdown?
How about we just stick with HTML?
If I ever decide against Markdown, some sort of strictly-defined subset of HTML would probably be my second choice. As someone who's been building web sites since 1996, though, I find HTML unpleasant to use for writing prose, and less readable when unrendered than Markdown.