Markdown can work more like your mind. Restructured text is still too much like programming.
I see markdown more for writing a quick note or something that will not exceed a few pages.
Other light markup formats were designed by programmers, primarily for computer consumption, with readability by humans as a secondary goal.
It's a people problem, not a tech problem. I love writing in markdown because when I write, my mind works differently than when I code.
Ideally I could do anything that sort of feels right and the computer will just figure it out.
Markdown is the closest to that^ that I've ever used. It gets out of my way, which is what I want from a tool.
The lack of pure fancy spec is a feature
(inb4 "But my use-case is different." Yes it is, but I'm talking about the web in general.)
Never use underlining, unless it’s a hyperlink.
[1] http://practicaltypography.com/summary-of-key-rules.html
Markdown is generally less expressive (I would love if MD had RST's "include" directive) but I find it much easier to format a doc in MD.