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.