I get the feeling Glenn Kenny has hit the nail on the head:
The business in the film about "shared dreaming"—about which we are told very little except that in this film's world it does, you know, exist—is merely a trope, one that enables Nolan to play what you might call a game of multi-dimensional narrative chess. And it is at this level that the film really engages. And that ain't nothing.
(I haven't seen 'Inception', but.) Nicely distilled.