.. extension:: hello world
In markdown, on the other hand, you have multiple, incompatible versions which have entirely different syntax because there is no generic extension mechanism.
ReST feels more well thought-out, generally.
That said, I've pretty much given up advocating it, because markdown seems to have won and has so much more tool support.
It's more powerful and looks much cleaner
// e.g. how do you write footnotes in markdown? And how do you do this in markdown?
+------------+------------+-----------+
| Header 1 | Header 2 | Header 3 |
+============+============+===========+
| body row 1 | column 2 | column 3 |
+------------+------------+-----------+
| body row 2 | Cells may span columns.|
+------------+------------+-----------+
| body row 3 | Cells may | - Cells |
+------------+ span rows. | - contain |
| body row 4 | | - blocks. |
+------------+------------+-----------+[^1]: My footnote here
Here's a link to pandoc's markdown flavor, which includes tables: http://johnmacfarlane.net/pandoc/demo/example9/pandocs-markd...
It's certainly set up for extensibility, although some areas are a pain (e.g. blank lines may be ignored entirely, change vertical spacing or change semantics, that can be annoying).
The toolchain is also fairly complex and very badly documented (Sphinx itself is better documented than docutils, but either way you end up wading through piles of code to understand what happens, why, and what's available in the applicative context of your extension)