Good commentary, but renaming it entirely seems a bit strong to me. It is, or intends to be, a better spec'ced version of Markdown.
I don't disagree that it is still mostly Markdown. I do understand the Gruber complaint about it not being a strict superset. I see it as similar to C/C++. Not all C code is valid C++ code--they have diverged. No one would deny that they have a very closely shared heritage.
C++ added features to C. Common Markdown's main purpose is not to add features to Markdown; rather, its purpose is to standardize it:
> We propose a standard, unambiguous syntax specification for Markdown, along with a suite of comprehensive tests to validate Markdown implementations against this specification. We believe this is necessary, even essential, for the future of Markdown.
I think the Common Markdown community may (and should) add a grammar at some point, based on some discussion two days ago.
On what grounds? I do not believe the legal situation is that clear-cut.
As far as the code goes, copyright is not involved as they don't use his code. The trademark hasn't been enforced and there are no trade secrets or patents involved.
The only legal issue I see involves the format itself. I'm not sure what US laws say about that, but I'd be surprised if there was an issue.
Gruber has a registered trademark or has he used the "TM" designator to denote a non-registered but protected trademark? My understanding is that if he has done neither, he doesn't have a legal claim to the name.