As in Memento, Nolan juggles layers of consciousness better than almost any contemporary writer/director. Inception is comparable to works by Kurosawa, Hitchcock, even Goddart & Bergman at offering immersible dream states as locations for exposition and resolution.
By teaching his new architect (Ellen Page) to pay attention in the dreamstate, DeCaprio's character actually teaches the audience to do the same. It's being conscious of where you are -- at what dream level and with what time deadline -- that give the narrative its drive and the action a tight grip on our galloping hearts.
Nolan nearly bit off more than he could chew, as evidenced by some in our viewing party leaving the theater not really sure of what happened. But both high school teenagers and middle aged film buffs had a grip on the plot points, meaning Nolan left enough bread crumbs for some of his audience to pick up.
In exhorting her colleagues to go one layer more, Ellen Page's character proved an essential hero and a mighty helper to the mark (Cillian Murphy), the client (Ken Wantanabe) and her new boss (DeCaprio). Her insights were keen -- we all could use a sidekick like hers.
Note: Did anyone feel that DeCaprio's yearning for his (dead) wife and his (living) kids had an odd parallel to his character in Shutter Island? Both had subconscious guilt about a personal failing that might have caused the loss of a beloved family and spouse. Here, DeCaprio (with Page's help) stays focused on the real; in Shutter Island, he cracks into the broken plane of the unreal.
Perhaps a more compelling reason for the client's initial hire -- corporate espionage is real enough, but to put the teams lives on the line in situation after situation, is breaking up a monopoly a reason to risk seven lives? Nolan thinks so.