I am not sure I am comfortable with the idea of an entirely context-less "bad concept". I have the annoying habit of answering questions of the type "Is it good/bad, useful/useless, etc." with a counter-question "For which purpose?"

Yes, I understand that rare pathological cases should not crowd out useful generalizations. However given the very strong implicit context (along with the whole framework of preconceived ideas, biases, values, etc.) that people carry around in their heads, I find it useful and sometimes necessary to help/force people break out of their default worldview and consider other ways of looking at things. In particular, ways where good/bad evaluation changes the sign.

To get back to the original point, a concept is a mental model of reality, a piece of a map. A bad concept would be wrong and misleading in the sense that it would lead you to incorrect conclusions about the territory. So a "bad concept" is just another expression for a "bad map". And, um, there are a LOT of bad maps floating around in the meme aether...