YAML and Markdown are important

But both could be improved upon, and should be improved upon.

They are important, because in this world there remains to be seen a data-entry tool more powerful than a keyboard, which with it's ≈100 keys can enter as many as log₂(100) ≈ 6.5 bits of information per keystroke.

Further, there remains to be seen a more powerful paradigm of exploiting the keyboard than the advanced text editor — Vim, EMACS, SublimeText, (and even the X window manager's XCompose functionality.)

Markdown and YAML both capitalize on just that: they are formats of structured plain text that favor ease of creation. (It is in the nature of plain text that ease of management comes packaged with.)

Markdown has an enormously low cost of entry: start typing. And whenever you want something more advanced, it lends you the less often used punctuation marks for it: asterisks, number signs, underscore, greater than, and others, in order to specify special meaning. And when you don't want these symbols to be special, you can escape them with the backslash (which is only a keystroke away to US-layout users.)

Markdown's weakness lies in it's closeness to the HTML format, in the fact that there until very recently existed no standardized specification, and in the fact that it has a couple of different syntaxes for each kind of semantic element which complicates parser-writing.

YAML is popularly called a successor to JSON, and is indeed a proper superset of JSON (meaning all JSON is valid YAML.) YAML, however favors a line-oriented, indentation-based syntax which is substantially easier to read and manipulate, and further offers a few different syntax options to make reading easier — overall, YAML reads like data structure pseudocode.