The biggest thing I took away from Longinus' essay On the Sublime was the part when he addressed the "five most productive sources of sublimity" (Longinus 350). The five sources are as follows:
(i) The first and most important is the power to conceive great thoughts...
(ii) The second is strong and inspired emotion. (These two sources are for the most part natural; the remaining three involve art.)
(iii) Certain kinds of figures. (These may be divided into figures of thought and figured of speech.)
(iv) Noble diction. This has as subdivsions choice of words and the use of metaphorical and artificial language.
(v) Finally, to round off the whole list, dignified and elevated word-arrangement.
I found this to be the most important because it was the prime example of Longinus explaining what is needed for a piece of writing to be useful.
He explained this on the first page by saying "two things are required of any textbook: first, that is should explain what its subject is; second, and more important, that it should explain how and by what methods we can achieve it" (Longinus 346). Longinus does the former easily in the beginning of his essay and achieves the latter, more important one, in the five steps that he explains that I also listed above. I wanted to touch on this because I do think that the two things Longinus mentioned are crucial but I also think there is one more important aspect. I think the final important aspect needed is that both of these things need to be clear to the reader. This is where the writer-reader relationship comes into play. For example, a writer needs to make it clear in his/her essay what his topic is but also make it clear how that certain topic can be achieved. I think that is something Longinus did very well. He made it clear what the premise of his essay was and also made it abundantly clear how that could be achieved in the five steps that I quoted above.