I'll explain my feature request with an example.
If you have c++ in your favourite tags in SO, then automatically c++11 tagged posts should show up shaded on your front page. Because any C++11 question is obviously also a C++ question.
I think there should be a feature to mark a tag as a child tag or sub tag of a parent tag.
Another thing is that a c++11 may be interesting today but 3-4 years down the line when most C++ compilers are C++11 complaint, then a tag like C++11 may be irrelevant - so searching under the c++ should also search implicitly amongst question tagged only as c++11