There is a type of puzzle that has been created called 'theorems', which are puzzles that give a seeming nondescript image and a cryptic note, leaving you to find the phrase that is the answer (for a more descriptive version of the rules, go here.)
This type of puzzle was the reason I came to Puzzling, so I was disappointed to discover that the tag for it did not exist. I built up my reputation so that I would be able to create it, but the more I learned about the site, the less certain I was that the tag would be a success, since:
It could be seen as an advertisement. The website I linked to was the official website of the puzzle's creator, but he is trying to sell his books of the puzzles, so it could be viewed as an advertisement to create puzzles like them. This is not the intention. I just think they are fun to solve and create, so I think we should be able to share them on this site.
Not enough of the community in Puzzling could know the rules. This is one of the biggest problems that could come up with the tag: not enough people using it. I have no earthly idea how popular the Master Theorem book is, but considering the fact that no one else has created the tag yet, it might not be popular enough to support as a Stack Exchange tag. In addition, the amount of people who create questions in this format could vary widely, so I can't predict how it would fare in the community.
Some might confuse the name for a math situation, not a puzzle one. This probably won't happen, but there is still the ever-present risk of someone not being able to look past the name and closing the tag on math-related grounds.
I believe that if all of these situations could be avoided, this category would be a brilliant addition to Puzzling.
But I still need to ask. All things considered, should I create the 'Master-Theorem' tag?