Note that this article compares Pandoc Markdown to ReST; Pandoc adds a lot of features that vanilla Markdown lacks.

Of course, this is one of the big problems with Markdown; there are a bunch of different implementations, each of which adds its own extensions. Actually, if you take a look at the Pandoc homepage, you'll see that it implements 5 different Markdown flavors: http://johnmacfarlane.net/pandoc/