Inception/Analysis

Dream or Real World?
After seeing the movie more than once, the leading theories become more reinforced:
 * If one believes it is all a dream, that is evidenced by: the lack of explanation for Mal being on the balcony opposite their suite when jumping, Cobb's father telling him to, "come back to reality," the contrived coincidences of the chase scene, the random narrowing of the buildings in the chase scene, the fact that Mal, short for Mallorie, is also used in "Grand Mal" and Petit Mal, that mean Big Bad and Little Bad respectively types of seizures, and is pronounced the exact same way, the at times arbitrary rules of the universe, the top not stopping at the end, etc.
 * If one believes the real world was indeed real, that is evidenced by: In the dream world and in memory, Cobb is always wearing his wedding ring, never sees his children's faces, and they are always wearing the exact same clothes. The top also never stops spinning of its own volition. In the real world and present time, Cobb's ring is absent, and when he finally sees his children's faces they are wearing subtly different clothing—the plaid of James's shirt is no longer diagonal and is less thick, and Phillipa seems to be wearing a white shirt under her similar dress. Cobb's personal totem seems to have been his wedding ring, and when he spins the top and lets it go without waiting to see if it stops, he is consciously relinquishing the cancerous idea that took part of his life from him in order to be there for his children. While the trope is brilliantly played with, and the viewer is meant to wonder, many theories about the movie pay a bit too much attention to the tricks of the genre and don't take into account the emotional heart of the movie, which is Cobb's love for his children.

How does Fischer come out of Limbo safely while Saito does not?
The circumstances of death are different. Fischer dies of a mere gunshot wound. He dies in a way where he can be brought back (hence the defibrillator). Fischer is very close to Limbo, but not quite there, because he isn't truely dead yet. This is what allows them to enter the Limbo-esqe dreamscape; Fischer is nearly there.

Thats also why the infinite time dilation hasn't kicked in yet, it doesn't become full-on Limbo until you reach a point where you cannon come back to life. Because Fischer could still be revived, we only got a dreamscape shaped like Limbo. And because ideas like this exist in each of their heads (whether they realize it at the time or not), and because of the Your Mind Makes It Real aspect of it all, this is certainly plausible.

Meanwhile, Saito dies of a punctured lung, on top of drowning. There is simply no way the body can be revived after that, which is why he was in an actual Limbo state by the time Cobb got to him. Again the your mind makes it real aspect comes into play, everyone knows it's pretty much impossible to recover from that. That makes it so.

Also, because Saito is now dead in all three levels, trying to wake him up from a level three Limbo would result in him going to level two. In which he is also dead, thus sending him back to Limbo. He would need to be kicked from level one limbo to get out. This is why Cobb goes back to level one and dies there (though he really dies in all three levels simulaneously), so he can get to that level one limbo.
 * How does Cobb get back to Level One, since he's missed the chain of kicks and the chain of worlds has been broken?
 * While Limbo is described as being very 'deep', it's also described as 'unconstructed dreamspace'. Therefore Limbo is always what's directly below the last constructed level of a dream - whether that's the third level or the first level. When Eames (3rd level dreamer) and Arthur (2nd level dreamer) were 'woken up', their constructed dream levels crumbled, which means that Limbo is the level immediately below Level one and technically Saito and Cobb only need to kick up one level to get back to level one.
 * ...or does he?