Discussion: What criteria should determine inclusion into the warn menu?

[Amorymeltzer]

Additions to the twinklewarn menu are commonly suggested, and right now our inclusion criteria are a bit ad hoc, so it'd be good to nail those done, especially for #477. As @atlight nicely summarized, there are two major points we should consider as guiding concerns:

1. Length of the list
2. Not disrupting current practice

A swollen list makes it difficult for anyone, in particular newish users, to find what they want (#517 may help). We're kind of begging the question here, given that most warnings are likely given by Twinkle and Huggle, but regardless we do not want to overwhelm or aggravate antivandalism efforts.

#412 is probably where the most consolidated discussion has taken place, but as a PR has gotten unwieldy and isn't a good starting place, so I'm opening this to supersede and centralize discussion of the merits. The last major pruning was done in #302 by @MusikAnimal.