Rationale behind the creation of this group.
When used to one markup, being faced with a different semantic is annoying. E.g. surrounding a word with a single underline? Italics or bold?
When subtle (or unsubtle) variations catch a user out, annoyance can lead to lack of interest.
If a user expects tabular layout and finds it missing, or some other feature which they are used to, then using another, seemingly similar Markdown syntax can become fraught.
John Macfarlane explains some problems
Since Markdown(MD) isn't formally defined, syntax and semantics, any implementation is free to do as they wish. A result of this is that some people, who use different schemes, are tearing their hair out over what can be trivial, or could be major differences.
When I write an enormous table using one MD syntax, I'm never sure it will be viable on another implementation.
Extensions and how to take advantage
We all consider extensions, except there being no standard, there is logically no such thing as an extension to it? How might we incorporate extensions usefully?