Just finally saw this.
Read the other theories regarding level 1,2,3 etc.
Here is my take, based on a few odd things I noticed.
At the "crux" of the movie is Cobb's regret at having (perhaps ostensibly) performed inception on his wife. This regret is the crux because it is key to their troubles and failings, and therefore plot progressions, in each of the levels. Ariadne reminds Cobb of his culpability in that regard quite a few times. (you know.. that he should warn the others about his troubled subconsiouc) "regret".
Then, there is Edith Piaf's "je ne regrette rien" being played, for some reason, as the reminder to "kick out". Why does the tune happen to be THIS tune, given infinite other possibilities?
Also, because I happen to like that tune, I noticed early on that the tune is playing or overheard in every single "layer" or scene. It was also playing in what was supposed to be the 'reality' layer. This implies there must be some higher level where the actual music is originating. THis higher level is never shown on screen.
Additionally there is the sound effect of, what you might call, "ripping" or "unravelling", which seems to indicate that a dream state is about to, well, unravel. This effect is present in every level displayed on screen (including the final scenes which are supposed to be real), further suggesting that we never actually see reality.
My take is that Cobb is somehow trying to "inceive" into himself a way out of his guilt and regret -- guilt of losing touch with reality, or causing his wife to lose touch with reality. In this sense he is lost in a recursion, like a benzene ring. This is indirectly hinted at as him being stuck in a maze (aka created by obvious nomenclature with Ariadne) but it is also explicity demonstrated by the rotating stairs to no where.
Cobb is stuck in a circle of dreams, symbolized by the stairs to nowhere, in an attempt to rid himself of his guilt. We never see reality and Cobb will never escape.
-jM
A&D