Would a 'derivations' tag be useful? i.e. for questions asking about the derivation of established laws and equations. In my experience, understanding derivations (at least of college-level physics material) can be extremely useful and insightful---often far better than memorizing a result. A 'derivations' tag might be good to give these types of questions/answers more attention, and let people especially fond-of/expert-at them find them more easily.
My primary concern with such a tag might be that it's too meta. Being a derivation, however, does directly address the content of the question itself and the relevant skills/material required to address it.
An additional modifier might be good to distinguish "derivation" as in, 're-deriving an equation'; as apposed to derivative 'derivation', as in an anti-integral. Especially for non-native english speakers, this could be an issue.
This post ("Rigorous treatment of blackbody radiation"), for example, might be a good candidate for such a tag.