Léa Seydoux, who plays the Lady with the Blue Hair, is a marvel. Her performance, unlike that of Adèle Exarchopoulos, hints at a deeper backstory, one where she's spent much of her life attempting to cover up past emotional injuries. After three hours, it felt like I had only just gotten to know her.
That said, the movie could have easily been only two hours long. I actually sped up parts of the movie, particularly the ones where Adèle teaches children or stares off into the distance during parties, at double speed and did not miss anything. Turns out, watching a character do real life, every day tasks at normal, real life speed without any edits is pretty boring.
The narrow aspect ratio (2.40:1) is a missed opportunity. Director Abdellatif Kechiche and cinematographer Sofian El Fani love to to shoot their characters in extreme close-ups (ECUs). Traditionally, filmmakers tend to "save" ECUs for essential, movie-changing moments. Their overruse signals that the filmmakers may not have truly understood their story, since each moment is depicted as essential. There are great moments sprinkled throughout the movie, but I would have a difficult time pinpointing them because the ECUs inured me to their importance.
The sex scenes also feel like missed opportunities. They are long and sensual, for sure, but remember that this is Adèle's first sexual encounter with a woman. Wouldn't it have been compelling if she had to try to keep up with the older, more experienced Léa? With that approach, audiences would have better related to Adèle's experience. Instead, both women act like they are assured experts in bed, which is surprisingly boring to watch.
I agree with critics, such as Manohla Dargis, who argue that the sex is shot from the perspective of a straight man. The bedroom is lit brightly, so everything is visible, but it's not true to the intimate moment; the women moan loudly in the exact manner of porn performers faking moans. The two performers apparently felt very comfortable with each other on camera, so the errant approach to the sex scenes is all the more tragic and wasteful.