Divisions of Christianity should be our primary tags.
I've been using David Stratton's excellent answer as a touchstone when reading questions on the site. If you buy into the philosophy that we are looking down at Christianity rather than up for Truth, the largest regions of our map should be the three major traditions:
And we should have spaces for groups that fall outside of those categories and also tags for denominations within them. And there might be divisions that encompass several traditions (such as Chalcedonian) and others that cut across them (such as charismatic). In essence, the main tag should answer the question:
Which group of Christians is the question about?
Note that this could also include systems of belief popularized by individuals (see lutheranism and calvinism). But tags for individual people (martin-luther and calvin) should be paired with:
That tag is useful for questions that cover Christians throughout history; the largest division in many ways. Another tag that might be beneficial is the denomination for when you can't really identify who believes something.
Systematic theology items should be secondary tags
Besides individual Christians many of our other tags seem to be ideas that would make good chapter titles in a book of systematic theology. So the primary tag would tell you which volume to take off the metaphorical shelf in order to answer the question and the secondary tag would tell you which chapter. For instance:
However, a number of our most popular tags that are problematic. Many of them seem to be added to a question without any real understanding of how they might apply:
I think what happens is that people come to the site wanting to know more about Jesus and/or God so when they need to pick a tag they pick one of those and submit. That's fine, but we need to really be vigilant about cleaning up those questions. Otherwise, those tags become meaningless. (And there could be no greater tragedy, I think, than a site about Christianity where Jesus and God are rendered meaningless.)
I have strong opinions on the following tags that I will not reiterate:
Finally, I think the consensus on two meta-questions is that the doctrine tag is useless.