What fills the void left by the lack of character development in Inception?

I am not sure I agree that the characters are any less developed that many broadly similar movies.

Arthur is his partner - it is a professional business arrangement, he is being well paid to do this job. Eames is also being paid, and decides in the first dream level that it is not worth the risk - so clearly he is only motivated by the money. Yusuf is being paid twice to keep quiet about the dangers of the compounds he has made. Ariadne is being paid, is also fascinated by her new powers as an architect and has started to develop concerns for Cobb and his sanity. Saito is motivated by the power and money it will make his corporation. Robert Fischer as 'the mark' has the most complete back-story outside of Cobb and Mal.

How is this any different than other caper movies - Oceans Eleven, the Italian Job? Inception is very polished and has a ton of subtext thrown in, but substantially the characters have no more or less backstory than other movies about professional criminals.

In my opinion we are drawn in by a movie with very high standards of production, it looks great - the acting is good, the story is well paced and well explained despite its complexity, and we are drawn into the idea of double guessing whether Cobb is still in a dream or not at the end, and we don't quite know enough even at the end. Best big budget Hollywood movie in some time.

David Kyle Johnson, author of Inception and Philosophy: Because It’s Never Just a Dream, believes that the whole movie is a dream, Saito’s in particular and that the flat characters make for a better film.

Johnson and his collaborators sank considerable time and thought into their analysis of Inception and its implications; see Google tech talk and slides. My summary of Johnson’s justification for his position is below.

To start, subconscious elements work their way through dreams, e.g., the train barreling down the street in the rain dream, the random string of numbers in the hostage scene appears repeatedly in later scenes: the safe combination, the fake phone number, hotel room numbers. At the beginning of the film, Saito dreams of a mansion on an ocean.

Picking up on a big clue in the final scene may require turning on the closed-caption subtitles. Cobb asks his children what they’ve been doing, and they reply that they were building a house on a cliff.

Nolan leaves many clues that the film is a dream.

• Mombasa was like a maze.
• The walls close in around Cobb.
• Agents appear out of nowhere.
• Saito appears out of nowhere to rescue Cobb with a cheesy line about protecting his investment.
• Cobb’s father-in-law pleads with him to come back to reality.
• Eames is a dream forger who is able to pickpocket people without touching them.
• Eames bet his last chips in the real world but magically produced two stacks of chips to buy beers.
• Mal somehow got to the other hotel across the street in the suicide scene, but Cobb begged her to come back inside to his room, reasoning that would have “made sense” only in a dream.
• The top totem gives us no information about whether Cobb is dreaming because everyone else knows how it works.
• The Édith Piaf song that signals the end of the dream is 2 minutes 28 seconds long. The film is exactly 2 hours 28 minutes long.

When someone commits suicide in limbo, the subject goes one layer up. For Cobb, the next layer up was the snow fortress. But everyone was gone, so he filled the empty dream space with his own expectations, namely the airplane scene. Eames pickpockets the passport in the airplane without touching Robert, the way he did in other dreams.

This interpretation makes a much better film. Consider:

• All characters except Cobb are flat and one-dimensional — many didn’t even have last names.
• Editing in the real world jumps around without transitions.
• Saito swoops in out of nowhere with a cheesy line about “protect[ing] my investment.”

If the entire film is a dream, these gaffes become strengths. The characters are flat because they’re projections, not because the writing is bad. The jumping around is not bad editing but because that’s how dreams run. Saito’s cheesy line becomes a subtle clue that Cobb is dreaming.

^{The answer above repurposes content from an answer of mine on scifi.SE.}