Would someone please provide some highlights for how the Standard flavor is different from the Gruber/Github/StackExchange flavors? I'm sure there's a mountain of details that were previously unclear and now aren't, but as an end-user of Markdown, the web page doesn't make it clear what this changes for me.

What are the big things I have to start writing differently now? Have any of the "why can't Markdown just do X sensibly?" quirks (e.g., line breaks in lists, or what kind of brackets to use in links, or underscores in words) been worked out? Where am I most likely to encounter divergence (on something I actually care about) between StackExchange or GitHub markdown and Standard Markdown? What (if any) new syntax features have been added?

It's covered in this topic already:

What changed in Standard Markdown?

In general, as little as possible! The goal of Standard Markdown is not to redefine what Markdown is, or change the syntax, but make it parseable and predictable. We did try to address some minor persistent Markdown complaints we've seen from millions of users in GitHub, Stack Exchange, Reddit, Discourse, pandoc, and babelmark. John MacFarlane documents them here: In a few cases, I have departed slightly from the canonical syntax description, in ways that I think further the goals of mar…

Any specifics you have that aren't answered by that topic? You can experiment with whatever Markdown you're curious about with the Dingus at

http://try.standardmarkdown.com/