> For that, I think I want the current '|' binding to pipe the current
> message and then a new binding ("M-|" ?) to pipe every (open) message in
> the thread.
+1 

> Which makes me think that other operations should work similarly. '+'
> and '-' should change tags on the current message (as they do currently)
> and then new "M-+" and "M--" could change tags on all (open) messages in
> the thread.
+1 

> That would highlight the current 'a' as out of place since it's
> currently archiving every message in the thread. So I'd then fix it to
> be 'a' for the current message and "M-a" for every (open) message in the
> thread.

It would indeed be out of place, but I would like to voice support for
the status-quo. In General I like to press as few keys as possible
(since I use emacs my meta keys are getting worn out quite quickly).

and I usually archive whole threads rather than single messages, so I
would vote for "a" as archive thread. Is it inconsistent? Yes. Does it
let me press fewer keys? Yes...

Anyway, I can always customize it the way I like it best :-)

Sebastian