The story about von Neumann is a familiar and precious one. To me, a non-mathematician looking at mathematical writing, one of the most easily detectable changes in writing is that which takes place in the years roughly 1870 to 1940, when the last generation(s) of mathematicians who grew up with Greek and Latin were finishing their work. The classical heritage of writing gave people a commonality and a sense of responsibility in communication which is lacking in much of the writing of modern mathematics, where to write obscurely is often considered praiseworthy.
On Washington, it's worth observing that only two men who attended the Constitutional Convention did not have college educations -- Washington and Benjamin Franklin. It is also instructive to think of the mathematical attainments of that group, mostly attorneys. One thing they shared was having spent a lot of time with Euclid; probably their skills at algebra were not very impressive, especially by modern standards, but they probably knew considerable geometry and deductive logic. The heritage of logical thinking and writing, one might speculate, had quite an influence on the way the Constitution was written and what it said. One wonders if a generation trained with less depth in logic and rhetoric would have done so well.