Suppose one votes to close a question in "main", and the poster asks in "meta" why it was closed. Apparently the existence of that discussion in meta is not included among the "notifications" received by those who closed the question. Since it would be impolite for the latter not to respond if they knew the discussion was happening, this amounts to a bug ('bug' in the informal sense of bad design, not the technical sense of the 'bug' tag which refers to unexpected behavior).
In such a case it is customary to leave a comment on the post being discussed linking to the related meta question. This should usually work; and it directs further comments and discussion to meta.
There's little point to notifying a VTCer that the post is being discussed on meta -- s/he can't revoke the vote. As long a you get a good portion of the community discussing it at meta, it doesn't matter who is there.
Clarified suggestion in view of comments (for original see below):
As it is possible to notify various users that interacted with a post in a substantive way (commenters, editors, moderators that closed it) it could make sense to also be able to notify users that closed or reopened a question. This feature might be something that one could add.
To sort of answer my own suggestion. A counter-argument, as @all was removed and only one @ per comment is valid, might be that neither does one wish to have a somewhat bulk-ish notifier for all voters (of a given type) nor does one want to encourage five comments being posted to notify each individually.
Perhaps, it is not that feasible idea in the end. But, at least I learned something via my suggestion. Thanks to Manishearth and Kaveh for information.
While the question grew out of a somewhat complicated situation and thus might not be presented in an optimal way, I think it touches a very valid point. I do not fully share the specific suggestion of automatic notification when a meta-thread is created, it feels a bit complicated, might not always be necessary and is also somewhat limiting (see below).
The main issue it raises in my mind is whether who can be notified via an @ is sufficient/optimal? (I do not yet fully oversee this notification-system so I might say something that is not true, I welcome corrections.)
As far as I understand one can notifiy users that commented and OP is automatically notified. But, I think it could be useful to be able to notify everybody that interacted with a post in a substantive way, that is also users that edited, closed (does one now say, had put it on hold?) or reopened it. Or, to put this differently: users that show up in the revision-history of the post.
I can easily imagine situations where it could be useful to be able to ping somebody that made an edit to a post to get some clarification about a choice they made when doing so, for example. Also to be able to draw the attention from closers or reopeners to some debate could be useful, or also just to ask them specifically in cases of doubts what was the moticatiing reason could be useful.
I cannot really follow Manishearth explication that it does not matter so much who debates and also that people will notice the comment on main or the question on meta. By the same token on could question the usefulness of the entire notification system, since also a follow-up comment can just be noticed or answered by somebody else (and we lived quite well without notifications until very recently on MO). Yet, if one thinks that the possibility to notify is convenient (as I do), then I do not see why one should not also be able to notify a user that edited or closed (in addition to thos that commented).
Thus, I would like to present my slight modification of Michael Hardy's feature request:
Add the possibility to notify those users (via @) that show up in the revision history of a post (that is, users that edited or contributed to a completed closing or reopening).
I hope this would allow to address the problem Michael Hardy raised, and could also be useful in other ways, such as being able to ask for clarifications related to an edit.