Crossposting is frowned upon since the old news:// times.
However, between StackOverflow, SuperUser, and Serverfault there are some grey zones as this answer nicely highlights.
Of course, I have a particular case study for which I would get a wider exposure at StackOverflow, but hope to get a more professional view point here. Again, SuperUser could have the four folks who wanted to answer it, if they could see it.
So the question is either: Do we want to burden the answerers to create accounts at multiple sites and subscribe to multiple feeds to see all questions?
Or: Do we want to burden the askers to crosspost the same question to multiple sites?
The obvious follow-up: What happens if I do cross-post? Do the moderators on all sites have an agreement on how to handle grey zones? I doubt it.
To make it easy, I'd propose a feature to be able to create a "symbolic link" concept: Create a "link to a question" on another StackExchange site, that points to the original question and is indexed and searchable as if it was posted on both sites.
Care has to be taken to unify the tags database: Imho, they should be kept separate. But if a formerly unknown tag is attached to the linked question it could be imported as a new tag, complete with the tag wiki. On that other site, the new tag would list only the one question which just got linked to.
Should I crosspost this meta Q to meta SU and meta SO?