I was browsing through Jamieson's Rhetoric last night, a book originally published in 1818, and was struck by the final paragraph. Positively, because while it was meant to apply to the spoken word, it struck me as being perfectly applicable to the written word as well. But also negatively because the final word was too strong. Even assuming that people reacted far more strongly to mediocre art in the early nineteenth century than they do nowadays, it's an overstatement.
Also, I'm amazed that a serious rhetorician would end his book on a negative word. It seems a rookie mistake to make.
Nevertheless, as I said, applicable:
Finally. Guard against all affectation, which is the certain ruin of good delivery. Let your manner, whatever it is, be your own; neither imitated from another nor assumed upon some imaginary model, which is unnatural to you. Whatever is native, even though accompanied with several defects, yet is likely to please; because it has the appearance of coming from the heart. Whereas a delivery, attended with several acquired graces and beauties, if it be not easy and free, if it betray the marks of art and affectation, never fails to disgust.
Above: The title page. They don't make 'em like that anymore.