As we see, some interrelated SE sites like SO and programmers.SE has numerous common tags. As someone subscribes to a tag, it seems that that tag is interesting to him/her. So, we can assume that s/he will be interested to question tagged with that specific one regardless of from which sub SE site it is. But now the tags are site-specific, not universal. So, people are not being notified about new questions s/he might be interested in. And OP's are deprived of getting better responses.
So I request to make tag subscription universal throughout all SE sites. Or, the whole tag management(FAQ/Info/Top users) can be generalized to be universal within all SE sites. Then the users can get more about the things they are interested in.
My Filters
page. I'm not claiming that it meets your requirements completely, but it comes close and is already in place.