Markdown is kind of like CSV, where common usage pre-dated both Gruber's script and future attempts at standardization. His co-author Aaron Swartz confirms [1] that Markdown was greatly inspired by (and is largely compatible with) the way people used to 'mark up' plain text in ways that were emergent and natural to them back in the email days. Therefore, the lack of formal syntax is, to them, a feature.
This doesn't prevent forks, of course, but anyone who wants to perpetuate the 'Markdown legacy' should consider this.
I understand the appeal of basing Markdown's syntax off of the way people naturally tend to structure unformatted text (and in fact, that's one of the reasons I love Markdown so much), but I'm still really confused as to why anyone would regard syntactic ambiguity as a feature.
Leaving any sort of ambiguity in your spec inevitably leads to different implementations resolving those ambiguities in different ways, which confuses users and causes problems with interoperability.
Is there some sort of upside to these syntactic ambiguities that I'm missing which might offset those obvious drawbacks?
This doesn't prevent forks, of course, but anyone who wants to perpetuate the 'Markdown legacy' should consider this.
[1] http://www.aaronsw.com/weblog/001189