From GitHub Engineering:
[Markdown's] main limitation, however, is the lack of standarization on the most ambiguous details of the language. Things like how many spaces are needed to indent a line, how many empty lines you need to break between different elements, and a plethora of other trivial corner cases change between implementations: very similar looking Markdown documents can be rendered as wildly different outputs depending on your Markdown parser of choice.
This lack of consistency is incredibly aggravating if you use Markdown across a number of different tools.
For me, those tools include GitHub, Slack, Marked 2, syntax highlighters (for Sublime Text and Atom), WordPress, and now Ghost.
You surely have your own assortment of tools that use Markdown, and you know it is death by a thousand paper cuts.
For example (and not to pick on Marked 2), I've had instances where even with Marked's GFM parser on, there are a few Markdown nuances that will render differently on GitHub than in Marked itself.
And that's just for two GFM implementations. There's a wider Markdown world out there that's not even aiming at GFM.
If you depend on inconsistent Markdown implementations across your day-to-day work contexts, it means you've got to be aware of where you are writing at any particular moment in order to get it right 100% of the time. This is wasted mental state.
Will this new GFM spec fix the lack of consistency across the tools that we use daily? I'm not incredibly optimistic about it.
But I look forward to seeing if it has any bearing at all on the tools I use for work. There are certain tools I'd gladly trade in for others that get GitHub Flavored Markdown right.