Writing and speaking are such different media – for example, you can go back and reread a sentence, but you can't wind back time in a live conversation; you can signal how something should be interpreted,* give parsing hints, and add emotion using inflection and tone while speaking, but you can't do this in writing. So I think we should expect good writing to look different than good speaking.
PG might have in mind then the strange affectations that seem to grip people sometimes when they write, but I can't think of any examples off the top of my head.
It's not, I think, quite what you mean, but when you refer to "strange affectations" of writers (nicely put) I was reminded of this:
"Words resemble fish in that some specialist ones can survive only in a kind of reef, where their curious shapes and usages are protected from the hurly-burly of the open sea. ‘Rumpus’ and ‘fracas’ are found only in certain newspapers (in much the same way that ‘beverages’ are found only in certain menus). They are never used in normal conversation." - Terry Pratchett, 'The Truth'
PG might have in mind then the strange affectations that seem to grip people sometimes when they write, but I can't think of any examples off the top of my head.
*sarcasm, e.g.