is not a full sentence. It's shorthand for:
This is a great post.
That said, this structure gets used all the time in conversation:
Let's go bowling tonight.
It also gets used in print, when the author feels like the stripped down sentence might have more impact.
So, asking about "correctness" can be tricky sometimes.
I enjoyed not only what said about how that word should be written, but, especially, how all the matter has been treated.
Aside from the missing "you" (between "what" and "said"), I can't find anything else grammatically or syntactically incorrect, but it still reads a little awkwardly. I would suggest:
I enjoyed what you said about how that word should be written, but I appreciated how you explained the entire matter even more.
There's nothing wrong with using not only – it's a valid construct – I just felt it read awkwardly in your sentence. If you really wanted to use it, I'd move it to the front:
Not only did I enjoy what you said about how that word should be written, but I especially enjoyed how all the other matter was treated as well.
But that's a matter of style, not correctness.