There are discussions on giving warnings to those who creates new tags: 1, 2. It seems that this feature has not been carried out in Math SE. Does anyone know why? I would be very happy to see this happen.
On the other hand, I think we should also give warnings to those who only use rare tags (those tags, e.g. has only less than 500 questions) in a question. The wording would be like
Currently the tags of this question is not popular. Do you want to add more tags? A popular (and related) tags would highly improve the visibility of your question.
Given the number of questions we have each day, I figure out that I always missed interesting questions that tags (e.g.) only symplectic geometry without differential geometry, only (e.g.) calculus of variation without pde.