If you do much writing (or much reading) you’re probably aware that spoken and written language are two different animals. And even though we all speak our native languages fluently, we may or may not write that way. I know I’ve read some viciously bad writing in my time.
Basically, short, simple sentences not only sell more books, but spread ideas farther and faster.
If you do any writing, this is worth noticing. Of course, Seth Godin is always worth noticing, in my opinion.