I'd say expression coupled with insight and excellent taste is the sweet spot for the really good stuff. Putting together a great novel, say, is a lot more than knowing how to make yer words purty and fancy. It's also more than just telling a well-structured story. It's more than lifelike characters. It's more than prompting sharp and lasting emotions on the reader. It's more than atmosphere. Than having the confidence to let an amazing, beautiful idea slip by like a passing stranger's hint of perfume in a crowd. More than honesty. And so on. It's all of those things, continuously, for a few hundred pages.
[EDIT] though I'm not defending this letter as any kind of top-notch literature (is it supposed to be?). The parts I've read so far seem obviously too thin to qualify.
Yes. It's fashionable right now to be florid, but writers like Hemingway can punch you in the gut using simple, declarative sentences:
"This book contains material from the remises of my memory and of my heart. Even if the one has been tampered with and the other does not exist."
> For sale: baby shoes, never worn.
There could be some interesting ideas in there too, but that is not the main meat of literature.
I think its important for the sake of intellectual rigour not to confuse the two.