I've been reading a bit more online about Inception, and I wanted to scribble out some more thoughts.
1. Exposition and Plot Devices
The major critiques I've read of the film are it either spends too much time on exposition or that it doesn't spend enough. Complaints about the lack of exposition seem a bit unwarranted to me when the entire first act is devoted to laying out and explaining the rules of game. Other details, like how the dream sharing machine works and the background of Ken Watanabe's character, are essential to film and would have overloaded the film with expository dialogue. The dream sharing machine (dream machine? dream team machine?) is just a plot device, how it works doesn't matter to the plot and explaining it would have distracted from the neo-noir heist film milieu. As far as Ken Watnnabe's character, he night as well have been named Mr. McGuffin. His only purpose is to move the plot forward (instigating the whole "one last job" scenario, being the catalyst for the plan going awry, etc). His own motives, though interesting, are far less important to the film.
In regards to the film having too much exposition, I can only say that the rules of the game in Inception are relatively baroque and having them laid out through expository dialogue seems to have been the only way for the film to make sense on first viewing. Plus even though these early scenes were essentially characters telling each other how the rest of the film was going to work, they were visually interesting enough to be thoroughly engaging.
2. Narrative Structure and Dreams
A strange critique I've come across is that the film does not accurately portray how dreams work. Anyone who has seen Inland Empire could probably tell you why this is... movies based on dream logic aren't generally very compelling. Also, the film makes it pretty explicit that the "dreams" are just constructs designed to make it easier for thieves to access other people's minds.
I was actually really glad the film didn't expand on the dream thing any further. There was enough symbolism in the dreams that explaining things in greater detail would have opened the door for descriptions involving ridiculous Freudian or Jungian nonsense.
3. On the ending
The following may spoil the ending to the film. If you haven't seen it, I'd recommend skipping this part and just listening to Edith Piaf.
I've seen explanations of the ending ranging from arguments that the whole film was a dream to incredibly detailed descriptions of how the top was about to fall, just proving the final scene as reality. Here is my interpretation:
The last scene is an inception. Similar to Cobb's inception on Mal, the spinning top/jump to black leaves doubt in the viewers mind about the reality of the final scene... even though its actually real. The last scene being a dream just doesn't make sense from a narrative perspective (when did the dream start? why would Cobb suddenly change his dreams?). Even I, who likes when a movie has a downer ending, would have been left cold if the whole movie turned out to be a dream. Plus, I happen to think Chris Nolan is a better writer than to have a "it was all a dream" ending. I did like that it was left ambiguous though.
4. Secret Messages
I didn't come up with this, I just think its cool.