They describe limbo as "unconstructed dream space." In all the other levels, the different dreamers have had the environment constructed by the architect, so limbo is basically a level of dream space that has not been planned in advance, and is made up upon arrival. This is also why architects are so important; without someone to construct the dream, the dreamers would all just fall into this unconstructed space every time. As I understand it, limbo isn't a single level, but a term used to describe any lower unconstructed dream space. For example, while in Cob and Mal's limbo, one could drop into a further state of unconstructed dream space.
Here's where I get confused. When Fischer dies in the hospital level, and when Cob and Ariadne drop into limbo after him, they all arrive in what remains of what Cob and Mal built. Saito also dies in the hospital level, but he appears to bypass Cob's level, and go to an unconstructed level of his own. Since he appears to have aged significantly when Cob finds him, and has constructed a world of his own, it would seem to follow that Saito would have had to arrive in Cob's limbo, and die there, thus sending him to a further level of limbo. But none of that is shown on-screen. Saito should have simply dropped into Cob's limbo, but if he had he would not have aged so much by the time Cob finds him.
The only other scenario I can come up with, is that Saito does indeed wind up in Cob's limbo, but it takes Cob a very long time to find him. The trouble there is that Cob would have aged as well.
So how does Saito wind up in a dream for long enough to build a world of his own and become an old man, but Cob is able to find him without appearing to age at all himself?
Then there's the fact that Mal stabs Cob, but we never see him die, or in any real pain. Would that have woken him up, or sent him further into limbo?
As to how Cob and Saito get back having missed the kick, the last shot before they both wake up on the plane is Saito reaching for Cob's gun. Remember that to get out of limbo in the first place, Cob and Mal put themselves in front of a train. Going back to the idea that limbo is not a "level" so to speak that one must navigate up from, I would agree that, in the absence of dreamers in the other levels, dying in limbo is enough to bring them out of it.
This is, of course, assuming one accepts that everyone does indeed wake up, and that the end of the film isn't simply a further dream state. Nolan intended for the ending to be somewhat ambiguous, but I tend to fall into the camp that believes the ending takes place in reality. Sir Michael Caine would seem to confirm it as well.
(unless you were talking about the US version, then yes)
