To us, plain English is text that the intended readers find easy to read, easy to understand, and easy to act on. A suitable tone for the audience and purpose is part of plain English (and one of the key criteria we use when assessing documents). Structure is part of plain English. Layout is part of plain English. Elegance is part of plain English.
Plain English is the style of Abraham Lincoln, and Mark Twain, and Justice Holmes, and George Orwell, and Winston Churchill, and E.B. White. Plain words are eternally fresh and fit. More than that, they are capable of great power and dignity: "And God said, Let there be light: and there was light. And God saw the light, that it was good." (Joseph Kimble Answering the Critics of Plain Language)Joseph Conrad wrote in plain English, as does Kate Grenville. The American Constitution is written in plain English. Steve Jobs consistently worked magic with plain English. Plain words and plain meaning enhance the emotional impact on the reader. Nothing gets in the way of the emotional experience. ‘Plain’ is not boring; ‘plain’, well done, is beautiful.