I disagree with most of these changes. I feel the goal of Standard Markdown should be to specify the smallest agreed-upon subset of Markdown features coming from the original implementation, and that means not introducing features or changing semantics that better serve your own personal tastes.
Markdown almost always does need a number of extensions to better serve a particular scenario’s needs or tastes, and it is here that a standard base would be good: providing a clear basis in which to clearly and reproducibly specify where these extensions are made.
It’s fine to let different sites and implementations diverge from the standard, and not just because they already do - a site that needs tables, where the community is mostly coming from MediaWiki, will have a different call for table features and syntax than a site that, say, wants an easy way to format tables that are coming from a CSV (such as a statistics forum). Different places have different legacies, and different scopes of interest.
A Standard Markup should define the precedence and syntax rules for the subset of features that all Markdown implementations are expected to provide, that are as close to universally accepted as possible. If an implementation of a core feature defined by Standard Markdown is divergent in a Markdown renderer, it should never elicit a response beyond “oh, yeah, I’ll fix that bug”. If “Standard Markdown” dictates lots of unique and unprecedented quirks, it’s going to yield just another response of “oh yeah, that’s that site where they have all those kooky opinions, whatever,” and nothing gets fixed.
With a simple Standard Markdown base, changes to describe an implementation’s specific flavorings should be as stringently documentable as possible, with a series of “patches” that rarely, if ever, contradict something in Standard Markdown. Markdown implementations are then free to target one of these “flavors” as their own personal “standard” (or provide features of a flavor as options at render time, such as how https://github.com/chjj/marked handles auto-links and other GitHub-Flavored-Markdown features).