Things that come to mind:
* Inbox signature validation is very vague
* jsonld is a complex standard that introduces a need for libraries, leads to slowdowns and blows up the implementation surface
* Interaction schemes like quoting requests lead to nontrivial state machines
In general: any MAY in a definition explodes the possible things that can go badly.
Which is why I think we need to use a different approach from how e.g. RFCs are structured