But the best way to create an idea in a person's mind is through art.
So if "Inception" aspires to artistry—and I assume that it does—then it's attempting to do to us what Leonardo DiCaprio is trying to do to his dreamer. In other words, the premise of the film is actually a metaphor of the relationship between a film—any film—and its audience.
The question then becomes: What idea is this film trying to plant in our heads?
Unlike literature, unlike the theater, movies communicate their ideas primarily through imagery. Film is not language-driven. Despite its conventions of plot, dialogue, and narrative voice-over, film is much closer to photography than to the novel. To apprehend its ideas, we must contemplate its images.
There are some fantastic images in "Inception." Many of them you can see in its trailers.
Aside from the scene when a Parisian street explodes around our reposed stars, the best image in the film is the moment when DiCaprio and his wife rest their heads on a railroad track and await an approaching train. The camera shows us a rusting steel rail trembling against its spikes. Marion Cotillard's hair blows in the wind driven in advance of the train.
In the end, however—as with "The Dark Knight"—the images in "Inception" never coalesce into a forceful idea. The thought we're left with is is not really an idea but a question: What the hell is going on? All the fancy imagery seem to exist for Christopher Nolan not as approaches to metaphor but as a way to play with cool toys. The images are curiosities. They are exercises, in the end, in cinematic vanity.
So perhaps that's the idea: We live in an age of explosive vanity. Better, in the end, to live in our dreams.