Pithy always seems to me like a
bland, vaguely negative word; it reminds me of the
pith in certain plants, which is basically
organic styrofoam, with no real substance or texture or interest at all.
But, at least when the term is properly used, pithy when applied to a piece of writing is a high compliment, indicating that the language is simple and direct,
without fluff or decoration, packing a maximum amount of meaning into a minimum amount of words. Pithy writing is the verbal equivalent of Saint-Exupéry's classic definition of good design, in engineering or other fields:
"Perfection is achieved not when there is nothing more to add, but when there is nothing nothing more to take away."