Basic rendering is almost often wrong : lists, titles, hyperlinks. Instead of adding features, rendering engine should be fixed to work according to specs.
See Darling Fireball documentation. It is written in Markdown and Id the definitive Markdown test document. Web version is very different than the one seen in this app. Most of the issues are around markup that begin on a new line and that must not be considered in the same paragraph as previous line or vise versa.
I bought this editor and Im still waiting to be able to use it.
Online help on markup language should be available.
Web links and images should be visible and usable in preview mode.
micrond about Byword, v1.1