In terms of the plot, and what exactly happened at the ending, the movie is not that difficult to figure out. But I do find it interesting that a deep movie actually has a happy ending. More often, movie makers think that a movie just isn't intelligent enough unless it's a tragedy. Nice to see Nolan break the mold.
I think it's also worth explaining some of the whole "shared dreaming" business. It's a wonderful plot device--a bit like learning the rules of a game. But it does take some thought.
There are a minimum of three roles involved: The dreamer, the architect, and the subject. In our own dreams, we perform all three roles ourselves. In the film, those three roles can be performed by different people within the same dream. In a nutshell, the dreamer is the one whose dream others are sharing; the architect imagines a scene where the dream takes place; and the subject is the one whose subconscious populates and inhabits the dream in the form of dozens of anonymous people--"projections"--who basically exist to make things look normal.
Some analogies to help understand this:
If shared dreams were dinner parties, the dreamer is the host; the architect sets the plates and cooks the meal; and the subject is the guest and all of his imaginary friends come to dine.
If shared dreams were tennis matches, the dreamer is the sponsor of the match; the architect is the referee; and the subject is the tennis player and his imaginary opponent and/or teammate.
If shared dreams were paintings, the dreamer's dream is the canvas; the architect is a landscape painting; and the subject is the figures painted into the landscape to tell the story.
So in Inception, some guys try to perform corporate espionage by sharing a dream in which the mark is the subject--i.e., the mark's subconscious populates a scene invented by their own architect. It's as if the mark was acting in a play and not realizing it, and the play's director and other actors were trying to get him to reveal a secret.
I think it's a little unfortunate that Cobb's wife, Mal, is portrayed as the interminable dupe. For a dead woman she gets really poor treatment.
Mal, we learn, was fooled into confusing Limbo for reality. Then we learned that Cobb tricked her in to knowing the truth (quite a paradox, telling a lie to persuade somebody of the truth), which freed her from Limbo, but then led her to believe delusions in the real world, leading her to kill herself. From then on, we are left with an exceedingly needy subconscious projection of Mal, who we then pity because she is too gullible to know that she is a projection.
I am a little bit surprised that a contemporary Hollywood film was able to slip by the good-taste censors a depiction of the female lead as easily bamboozled, clingy, and helpless. It's like a throwback to pre 1950's depictions of women as "silly" or "hysterical". In that respect, Mal and Ariadne are foils for each other, but in an unsubtle way.
And yet still I see the irresistible romance between the early Cobb and Mal. They had a tremendous paradise together. They grew old together. They lived as gods within a world of their own making. Perhaps there is a kernel of that romance inside of every budding marriage; the almost intimidating potential of the new; the taste of paradise; the delight of being with one's beloved.
In different ways, both Cobb and Mal's insatiable desire for the everlastingness of their dream sabotaged itself. Mal lost herself in beautiful but false Limbo. Cobb was careless in his effort to retrieve her and so effectively killed her. Both sought to continue the permanent romance, each on his and her own terms, and in so doing botched it. A true retelling of Adam and Eve.
Is there a moral to the story? With respect to the tragedy leading to Mal's suicide, the film is fatalistic--it could not have been otherwise. All the remains is for Cobb to have a moment of catharsis and liberate himself from guilt.
But is there any way the paradise of the early Cobb and Mal might have continued? What if Cobb had relented and not insisted that they leave Limbo? Perhaps Mal would have come around on her own, though it might have taken another 10, 20, or a hundred years (time passes more slowly in Limbo than in real life).
Perhaps the film is right to be fatalistic. The danger of Limbo is that it is a paradise of human imagining. Where it does not terminate in monotony and sloth, it ends in addiction and infatuation. Cobb's and Mal's different Limbo experiences represent the different ways that all finite paradises are slated to fail--how not even immortality can rescue such a dream from entropy. Cobb became impatient with it. Mal became infatuated with it. Cue the disintegration of love into demands for control, hoarding, possession, and desperation.