That's just one of the positive aspects of Lynne Ramsay's direction, and I mention it first because I don't want this review to seem too negative, hence my burial of the next bit in the fourth paragraph, which the tl;dr generation probably doesn't even realise exists.
If you are still paying attention, then you're probably the kind of person who will find Ramsay's constant in-your-face symbolism as annoying as I did, i.e. quite. Tilda Swinton spends most of the film trying to clean red paint off her fingers: could it be that she feels like she has someone's blood on her hands? YES. A snapshot of a happier time in her life takes place in front of a massive UPS truck: could it be that this was one of the "ups" of her life? YES. The soundtrack is riddled with songs about mothers or sons: could it be that this film is about mothers and sons? YES, WE GET IT. If the film was any more heavy-handed it would need anti-gravity gloves.
And the red! There's SO MUCH RED in this film. I'm not thick, I know why it's there, but at times Ramsay's insistence on cramming as much blindingly obvious red stuff into her frame is in danger of becoming distracting:
Ramsay has, by and large, done an amazing job here, and We Need To Talk About Kevin will easily be the film on everyone's lips, rather than the book in everyone's hands, come awards time. Whether or not the whole world talking about it makes you want to see it is a different matter, but you can take it from me that you really should.
UPDATE: I wrote this review after several glasses of wine, and reading it back the next day I note that it dwells perhaps a little too intensively on the negatives. Frankly though, I can't be arsed to rewrite it and I'm quite pleased with the anti-gravity gloves bit so you'll have to put up with it. Just to clarify, the film is brilliant. Sorry.