John,
I haven't had the chance to (per)use the v2.4 directory mode in enough depth to contribute pertinent opinions. Just a problem of timing on my side, will catch up later, yet well aware that I am missing this one big moment while it happens.
Since I don't have much first-hand thought-out comment on your summary, I'll just footnote a minor nitpick ;-)
> For example, there does not appear to be any application that associates a directory
> extension and will automatically launch if such a named directory is "opened".
There is, technically, the shell itself - see the 2nd paragraph in http://www.ztw3.com/forum/forum_entry.php?id=95130.
That said, I'll agree that the above is just a technicality, most likely not used by (and even if, transparent to) most.
Cheers,
Liviu