Some hated its general tonal unevenness, blamed on the fact that it's a hybrid of Stanley Kubrick's starkness and Steven Spielberg's love of spectacle. This, in itself, isn't a sin, but because of this unevenness, the film requires total commitment and immersion from the viewer. As with other towering, ambitious stylistic films (Snyder's 300 being an example), any detachment will destroy the engagement, causing the film to alienate the viewer instead of involving them.
Other critics thought it was simply too emotionally manipulative, which, in a film like this, is something that may simply have to be forgiven... both Kubrick and Spielberg are dramatic stylists, melodramaticians, in a certain way, and the film itself is a tortured exploration of desperate childhood longing and the core of the human condition.
The most common specific complaint, though -- and the one that's most vulnerable to an actual analytical response -- is the hatred for the film's ending. Many viewers, including very open-minded ones, and including ones who saw it in the theater, felt that the ending was truly unnecessary and excessive, like a coda that somebody attached to the end of the film to make it more family-friendly. Many say it should have ended when David was at the bottom of the ocean, gazing at the Blue Fairy, wishing to become human.
I'm going to argue that the far-future ending wasn't just appropriate -- it was essential to the structure of the film, turning it from a tortured drama of frustrated longing into what it was truly meant to be: a dystopian fairy-tale in which every act is answered and every movement is integrated into the whole. The ending, though it may have been unexpected, unfamiliar, and incongruous with the preceding events, was also what elevated the film into something that will stand the test of long-term critical scrutiny.
(WARNING: Spoilers for Brothers' Grimm stories, in case you missed out on them for the last 150 years)
David's passage through the collapsing modern world in search of the key to humanity is famously analagous to Pinocchio's journey through the hazardous world of adults to eventually become a real boy. Now, Pinocchio was originally written by Carlo Collodi, was an Italian scribe who learned his craft with the translation of fairy tales, and the feeling of these fairy tales is impressed upon his work. This includes the sense that the world outside the home is dangerous and unpredictable, with no real place for heroism and personal glory. Indeed, the lack of a hero might be one of the strangest aspects of traditional fairy tales, like those of the Brothers Grimm -- the protagonist is always a commoner whose humble virtues are honesty and a sense of duty. These protagonists never discover that they're exceptional, and they never find some kind of inner strength to save the world.
This is a starting point for understanding where A.I. is coming from. David may indeed be exceptional in a scientific sense, but he isn't a hero. In fact, one of his greatest challenges is facing the fact that he's a manufactured creation, no more worthy of love than any of his boxed-up brethren.
Most of the Brothers' Grimm protagonists DO overcome some sort of personal challenge. Hansel and Gretel defeat the witch by taking advantage of a golden opportunity to push her into her oven; in The Robber Bridegroom, the bride arrives at the groom's house when he's not there, and she finds sympathy in the old woman whose head always shakes (damn, what a troubling image...); the prince, through wit and observation, finds a way into Rapunzel's room to win her love; the queen convinces Rumpelstiltskin to wager her child, instead of simply taking it from her.
Each of these represents some willfulness on the part of the protagonist to oppose their fate, and it is the same with David: his search for the Blue Fairy, though it means everything to him, is ultimately a token gesture of agency in the midst of an impossible situation. And yet, none of the above moments of triumph is really a solution, because each of these characters is stuck in a greater bind: Hansel and Gretel are lost in the forest, the bride is caught in a marriage to a criminal, Rapunzel is still in the thrall of an evil sorceress, and the queen is still caught in a promise to a tiny, evil man who's great with a loom.
But the thing is, each of these characters DOES find a happy ending, and it's not because they're heroic -- it's because of the grace of nature, of forces larger than themselves. It's because, when they remain true to themselves, they are rewarded with an escape from the impossible situations they're part of. Hansel and Gretel somehow find their way home (there are no breadcrumbs to guide them), and they discover their mother is dead; the bridegroom, by pure luck, acquires proof of her groom's evil deeds, and once she finds her way home, she is protected by her community; the blind prince stumbles across Rapunzel, and in what seems to be a miracle, her tears of sadness cure him; and after the queen is given Rumplestiltskin's name by a messenger, who was in the right place at the right time, and once she has defeated the little monster, she simply stands by and watches him destroy himself.
This moment of grace is essential to fairy tale structure. It's a message that's both hopeful and frightening: when you've done all you can to solve your quandry, you just have to hope that the fates smile down and solve it for you.
And this is what happens in the bittersweet ending to A.I. Humanity might have collapsed, but through their creations, these strange machines that exist beyond the singularity, their spiritual nature has been preserved by the universe, and they finally give David the happiness that he deserves because he so kindly and whole-heartedly desires it. There's also a mythical point here, too: the point that David is the Adam to these robots, that his first glimmer of human emotion is what has allowed them to evolve in the way that they have. However, this wasn't particularly an accomplishment on his part -- he never asked to be the beginning of a new cybernetic species. He's granted mercy simply because he's a thinking, feeling creature, filled with infinite longing, and that entitles him to whatever happy ending the universe can provide.
If he really had been stuck at the bottom of the ocean, which no broader, more transcendental resolution, wouldn't the tale have felt incomplete? Wouldn't the film then be saying that emotion, desire, and human frailty are anchors? That our longing, desperate hands reach for nothing, because there is no grace in the universe that we occupy? Wouldn't the frozen bottom of the ocean have been a truly chilling final image in this vast, cyclical film?
Personally, I would never have asked for any ending but the one I got.