I don't believe the real problem is grammar, spelling and punctuation, but something more along the lines of how to put together a convincing argument. The average student hasn't learned anything more complex than a five-paragraph essay. The ability to identify and employ the appropriate variations on that form is important for introductory-level work. Most freshmen are not up to the task (there was a kid in my own freshman English composition class who produced a seven-page paper of five paragraphs. reasonably smart guy, but clearly didn't 'get it').
Advanced work requires the sort of things I mentioned in the post--having read the work and thought about it and being able to say something that demonstrates a beyond-surface grasp of the material. That constitutes a reasonable standard for an 'A' paper, but you would be shocked by how many students cannot sustain that effort over five or ten pages. I'm not sure I would have believed it myself until I started grading.
Oh, well, in that case, count FLG among those with "little skill in English composition."