I agree in principle, but I think it may be a bit more difficult than it sounds.
Unlike Catholicism, LDS, and Jehovah's Witnesses, many "faith traditions" do not have a single right answer. For example, it can be difficult to give a single, correct answer to "what do Baptists teach about x?" Baptists are technically not a denomination but are a group/association/coalition of denominational and non-denominational churches, and only agree on a few points (and even then, that agreement is rarely unanimous -- some of them will even argue over whether Baptists are Protestants or not). For an example, see the answers to this question -- somewhat contradictory answers about Baptists churches that really mean nothing.
So before we go and close everything for not meeting objectivity standards (and that is the problem here; many traditions are assumed to be "normal" while others must specify their distinction), we need to actually figure out how this will work. Is a catch-all "Protestant" tag acceptable? The question would have to limit to the specific subset desired, but may (or may not) be better than three hundred tags like "southern-baptist," "american-baptist," and "first-baptist-down-the-street."
Instead of requiring a denominational scope without exception, is there any problem with specifying the belief itself in a tag; any denomination who teaches along the lines of that tag can answer according to their teachings. A question about the Trinity doesn't necessarily need to be tagged Catholicism or Lutheranism; it should specify "the Trinity according to the Nicene (or Athanasian) Creed." All Nicene Christians can answer this from their traditions, so does it really need to exclude somebody unnecessarily?