Adherence to the
Knox Decalogue?
But seriously, "mystery" is one of those loosely defined genres. It often overlaps with other genres like suspense and horror. What Hyouka does differently is to combine it with a kind of slice-of-life feel (another loosely defined genre).
Anyway, for me, classifying a work as a "mystery" depends on a few things. Firstly, it must bring up a question that needs to be answered, but it doesn't stop there. Next, it should revolve its plot around answering the questions it brings up. And most importantly, it must actively engage its viewers in the pursuit for answers by leaving clues and such.
That's my take on the whole matter.