I'll risk sounding stupid, but I'll say that there may be more than meets the eye on a first viewing with this particular film. I think only through a few rewatches may it really make a whole lot more of sense to people than on a first time. I don't think everyone got, or still gets Kubrick's 2001, or his entire filmography just by watching things once. I watched the Cannes panel for this film and a few other interviews with Dominik, and like Bourne already analyzed, there's more depth to this film, and each character represents a certain idea or archetype.
That being said...
Kermode also nails it pretty well with his review about how "everything is a transaction" there are no friends, or partners, it's all just a big business. If you want to go deeper with it. I'd say the film's intention is to show how Capitalism as a system, at least in America - has absolutely no morals about anything. The line about "...and in America, you're on your own" doesn't just mean you're on your own financially, and whatever holes you get dugged in because of the average being screwed, but "on your own" meaning at all, emotionally - there are no friends, even if they 'seem' like friends.
That's also why Pitt at the end said what he said, the last line "Now fucking pay me" - even though he was cracking jokes with Jenkins an hour earlier in the car, seeming like they're friends and partners - in the end, it's all a facade, everything. None of it matters - only money.
This is all the film's about in my mind. It's showing you the shallowness of America