Sapir spends much of the essay proving that English or French grammar is not as simple as is sometimes claimed. His examples are interesting, but if you have ever been caught out by English’s many homophones or heterographs, or the 13% of words that are not spelt as they sound, you probably did not need so much convincing!
For my money, the most useful section is:
“What is needed above all is a language that is as simple, as regular, as logical, as rich, and as creative as possible; a language which starts with a minimum of demands on the learning capacity of the normal individual and can do the maximum amount of work; which is to serve as a sort of logical touchstone to all national languages and as the standard medium of translation. It must, ideally, be as superior to any accepted language as the mathematical method of expressing quantities and relations between quantities is to the more lumbering methods of expressing these quantities and relations in verbal form. This is undoubtedly an ideal which can never be reached, but ideals are not meant to be reached: they merely indicate the direction of movement.”
His remarks that “A common creation demands a common sacrifice, and perhaps not the least potent argument in favour of a constructed international language is the fact that it is equally foreign, or apparently so, to the traditions of all nationalities.” is also worth understanding. Many IAL projects have floundered because they too closely resemble an existing natural language, with various historical, patriotic or nationalistic baggage.