Nothing happens
Before you jump down my throat and tell me I should stick to Transformers movies or car chases, here's a sample list of movies that I enjoyed with seemingly plotless plots:
Rachel Getting Married
Garden State
Synecdoche, New York
About Schmidt
I Heart Huckabees
The Master
I'm okay with slow, depressing, moody dramas but you have to remember to add a plot. Hypothetically, if there's a movie about a gambler who lost his winnings and his family left him, he needs to grow/change/make failed attempts at changing. He may or may not get his family back but the plot would be him trying to get them back. If there was a diagram, we could point to the precise points of his emotional growth which would either chart out an arc or an upward/downward slope.
Kidman's character is a flatline: she just sits and wallows in her own misery without any genuine attempt at changing. She's dragged to therapy and doesn't want to be there. She disregards her sister, her mother, her neighbors, and her husband. She strikes up a "friendship" with the boy who killed her son not b/c she wants to get to know who he is but rather who she wants to be. In fact, you could make a case that everyone around Kidman's character (Becca? too uninterested to recall) changes except her.
Yes, there are films that are heavy on character-driven stories but Rabbit Hole is more like character-driven stalking: the audience is on the outside looking in trying to figure out who these miserable people are and discover their motivations. But, alas, we never get past their character "types:" the angry husband; the miserable shrew of a wife; the underachieving screw up of a sister; the slacker boyfriend; the reserved mother; and so on and so forth.
Snapping polaroids of this couple's depressing life and then turning it into a flipbook is Rabbit Hole. There is a clever way to introduce the plot without spoon-feeding the audience but this movie fails to make any attempt at injecting plot whatsoever. What we have is what I saw and I saw nothing because nothing happened.
"Nobody can hear you. Nobody cares about you. Nothing will come of this."