This is of course related to the previous ones: Why not merge reputation across Stack Overflow, Server Fault, and Super User? Single location for SO/SF/SU user info
but not limited to that.
I mean, some questions clearly belong to more than one site, so it should work like "tagging". Probably my one isn't the best example of a question that belongs to two sites, but I think it gives you the idea:
I'm fine with keep things usually separate, but there should a way to have a question appear on more than one site, without duplicating it or migrate it (maybe just for user with a high reputation?)
feature-request? I ask only because I've been looking into the tagging system (and the tags here) and it strikes me that there is a lot of noise if you're looking for a real feature request... just say'n