Good question. I didn't see Juno. I loathed what I saw of Joss Whedon, so you're probably right there. But IMO the Coens are far greater artists than that. I don't have any unified theory of why, but I'll point out one thing. Lebowski has lots of zippy writing (different than Sorkin's, but I'll concede the point) but True Grit, to take the obvious recent example, does not. Neither did No Country. In other words the Coens have the discipline to subordinate their cleverness to the needs of the genre and the story. In fact this is one of their distinguishing features. Sorkin by contrast seems to have this adolescent need to prove how smart he is in every context, and is quite willing to turn his characters into caricatures in order to do it. It's a form of incontinence.
"Lebowski has lots of zippy writing...but True Grit...does not."
Did we watch the same movie? True Grit had some of the wittiest, zippiest dialogue I've seen in a long time. The writing was incredibly conspicuous, in a good way.
I disagree with your notion that conspicuous writing makes for a bad movie, just as I would argue that conspicuous editing, or score, or cinematography, or acting can lead to an excellent movie.
