sorry, I feel like I have confirm my translation of your intro paragraph  :  )
maybe it will help some others...


ended up with a hard dependency on this

my understanding is that you were depending on having PEP426 metadata, e.g. for build_requires.
since this PEP, as you say doesn't handle the "higher level problem" of specifying the types of dependencies (like PEP426 does), I guess you'll have another PEP in the works as well on top of this?  and then your build PEP would be depending on that.
 

bootstrap thing

you mean your other PEP idea for supporting any build system using the indirect mapping thing to processes...

 
'smaller step' or not

you mean Donald's idea of using "setup.py" as the build interface for now


Donald has graciously agreed to be a BDFL-delegate for it.

doesn't Nick actually don this delegation?... not that Donald wouldn't be great.  

 
and there are already existing
implementations of the dependency specification which we can instead adopt.
The existing implementations are battle proven and user friendly

for reference, which ones are those?


> The language defined is a compact line based format which is already in
> widespread use in pip requirements files

to be clear though, this PEP doesn't commit to how lines are put together in the metadata.   theoretically, this spec could be consistent with a higher-level spec that used json, right?  probably better to refer to "pip syntax" than "pip requirements files"