Roger Ebert sometimes talks about movies that rely on The Woman In Peril; when you need to ramp up the tension, do something frightening to the female lead. (I can't find the pertinent Movie Glossary entry. Fie!)
Juno is a screenwriter in peril movie. The witty script makes you identify with the writer, and the plotting is constantly skirting the edge of a precipice. Every major plot point turns on a scene that you know will be trite and predictable and much too long. And nearly every time, the scene unfolds along the lines you expected, but not quite that way — and it always veers away from the terribleness you expected.