When I ask a question and choose/define tags I follow the logic of keywords, but tags on this site are mostly used for categories that people want to follow or ignore.
I feel that this is not quite the same thing.
For example, in my newest question on the meaning of "zwie" in the word "Zwielicht", I finally chose as tags "meaning" and "wortteil" (=part of word).
But I feel that the question is somewhere in the middle between meaning and etymology and I am not sure that it is really of interest whether a question refers to a "single-word", "part of word" or "phrase".
So, which kind of tags are useful?