Thinking about it further though, this makes something of a nonsense of the original tweet, since it's hard to think what would count as "mostly original" by this standard. You might as well describe eg The Better Angels of Our Nature as "mostly not original" since it contains no original research but presents a synthesis of the research of others, building up to a common theme.

The problem I have is that if I say something that sounds positive about the Sequences, that's going to turn my friends off, since they already know I think well of them. By saying something that on first reading sounds negative, I might get their interest, but that only works if they go on to follow the link.