Juno starts off with dialogue so precious and seemingly contrived that I feared I was in for a depressing ride, but that's just a facade. In truth, the film could hardly be more sincere about its subject, ultimately evolving in a subtly cunning way. By the end, I found it rich and touching, in the good sense.
Ideally, all parts of the movie would be perfect, but assuming you're in for the whole picture, it's less important to start strong than finish strong, don't you think? And in the months since I first saw the film, I've come to think that the overarching dialogue at the film's start was designed to be slightly offputting, to set up a character still learning that wit is cool but insight and understanding even more so.