I read this entire thread, and the explanations people came up with answered most of my questions, although I'm not satisfied with some of them.
First, I think MrFang is right about the sedatives--the special strong sedatives have to be used in all levels, or else Fischer wouldn't have gone into limbo upon dying in L3.
Second, a bit of a plothole (or at least real misdirection), in the last scene, the top falling down doesn't even prove he's in the real world, because the top would only behave falsely (i.e., spin forever) if it was created by someone who didn't know the true properties of the top. But if Dom created this last dream with his kids and Michael Caine, his subconscious mind would create the top to be correct (falling)--he'd fool himself. A totem only helps you become lucid if you're in someone else's dream.
I agree that kicks produce merely the option of going up one level, rather than the compulsion to do so. Otherwise Arthur would have been forced to wake up from the hotel back into the rainy city upon the initial van kick.
One big problem I have with some theories is the time. People forget that even if you spend fifty years in limbo, in the real world, only a couple hours have passed. So, for example, choosing to hang around in limbo for decades has basically no (real-world) opportunity cost, beside the possibility of going crazy. The only reason for limbo is the special sedatives anyway--without them, limbo wouldn't even be a risk. (So for example, the crazy theory that Mal actually got out and is alive in the unshown Real Real World and is trying to convince Cobb to come out makes no sense; she could just wait a few hours, bam. By the same token, if Cobb really wanted to stay, he has no reason not to; upon getting out, he wouldn't have missed anything.)
Also, nobody's even questioned why Saito's much older than Cobb in limbo. Someone said above that it's because Saito died earlier But the difference in time would have been minuscule. L1-Saito would have been dead for a fraction of a second at most (in L1-time) before L1-Cobb's dreamself's dreamself's dreamself died. Remember, Saito died right after he threw the grenade. Cobb died a couple (L3) minutes later, either because L4 collapsed, killing L4-Cobb, or the mountain fortress collapsed, killing L3-Cobb. This would have been about 40 minutes later in L4 time, which wouldn't have been more than a day later in limbo-time. You could stretch the multiplier 300X and still only make that time difference a year.
Re: getting out of limbo, I'm conflicted. We know the special sedatives are the root cause of limbo. And it was explained that the fear of 50 years in limbo exists because outlasting the sedatives results in (e.g.) 5 hours of real time (the amount of time for the sedatives last) and multiplying it by 20^4 or whatever. So how do you get out of limbo?
1. Suicide or waiting out sedative (pro: explains train tracks double suicide, explains cobb and old-saito gun suicides. Con: Fischer did not kill himself, he got defenestrated by Ellen Page yet he still woke up)
2. Dying or waiting out sedative (pro: fits both Fischer and train tracks pair, con: Why didn't Cobb go into limbo straight from level 4 then? He couldn't have, or he'd have aged the same as Saito, and been there much earlier instead of arriving there on the beach 50 years later. Another con: Why would Cobb have needed to perform inception on Mal if he could have just killed her, then himself?)
Neither of these are satisfactory. Suicide is the easiest to make work, if you assume that Fischer wasn't really in limbo because maybe his L3 body was still barely alive, or maybe he got pulled into Cobb's L4-that-is-a-clone-of-his-old-limbo, or some other reason.