The distinction was important to me because I didn't feel the point of my original request had been taken at all, so I felt the need to be extra-clear about what I meant. The use of a different case for some letters is functionally distinct from the use of periods, commas, etc.

Whether capitalization falls under the general header of spelling or grammar I don't know—I think Phobos and I have actually argued about this, and there's a good case for either—but it's definitely not punctuation. By definition, punctuation is the set of marks that aren't letters or numbers; capitalization has everything to do with letters.

Really, come to think of it, I think you could make the case that spelling, capitalization, and punctuation all fall under grammar, but that's no fun. Also, if you're leaving a review or beta-reading, it's more useful to be as specific as possible. If I just say "some of your grammar needs work" when I mean "you need to capitalize the first word in your sentences, and also remember to use quotation marks around dialogue," how will they ever figure it out?

~Neshomeh