The most enduring lesson was an attitudinal shift toward the reader. The book's stance is that the reader is capable of and interested in understanding whatever it is that you want to say, and your job is not to dazzle her, or persuade her, but rather to provide her the conceptual building blocks to conclude what you have yourself concluded.
After reading the book I realized that, without ever having consciously decided to, I had been writing at least a little bit to signal how smart and creative and insightful I was, vs trying to communicate effectively. This was a revelation, and not a happy one -- so much misdirected vanity, to so little purpose!
It's too late to edit my comment, but as long as we're talking about it, I just read this article linked earlier today on HN, and it's such a lovely example of the classical style:
The author is clearly trying to give the reader what he/she knows about this subject, clearly and simply. It is straightforward, a delight to read, and an inspiration.
https://www.amazon.com/Clear-Simple-Truth-Writing-Classic/dp...
The best single book I've ever read on writing well. Had a big impact on me outside of writing, too.