- The music thing (from the YouTube above) is REALLY obvious the 2nd time around. You just don't think about it. Very clever.
- Limbo is indeed a shared dream state, just like the levels above. The only difference is that nobody has designed it, so all that's there are remnants from whoever has been there (Cobb). When Saito and Fischer "died", they went to limbo because the sedatives prevented them from waking up. Cobb and Ariadne went into limbo from sleeping in the last level because they had only designed 3 levels. So going any further than that meant they would go into limbo.
- The question of who's dreaming is actually answered in the dialogue. I was wrong in my initial belief that it was all one person; Dileep Rao's interview is accurate. It's Yusuf's dream first, which Fischer populates. They go to the hotel, which is Arthur's dream. The last dream is Eames's.
- The thing with defibrillating Fischer was kind of a cheat. You actually were just supposed to accept that a man could get shot above the heart, get defibbed, and then be ok. The defib served as Fischer's kick so that he would know to escape from limbo.
- There is a continuity error. When Cobb and Saito woke up, everyone else was already awake because the sedatives had presumably worn off. So it makes sense that Cobb and Saito spent more time in the dream state, and hence more time in limbo. Except the movie shows that they had already packed away the equipment, so Cobb and Saito couldn't have been sharing a dream. I'll attribute it to a production goof.
- Christopher Nolan drops some hints throughout that Cobb has lost track of reality. It's interesting though because in each case, the suggestion is given by Mal. She's the one that tells Cobb to forget the real world, and that he doesn't know what's real. She also tells him to take a leap of faith; that same idea (the exact line, actually) is used by Saito when he first offers Cobb the job. Then it's used by Cobb to pull Saito out of limbo.
- Then there's the top at the very end. Upon the second viewing, I'm going to say it wasn't going to topple.
- I'm not sure if this means anything, but I noticed that Cobb never sees his children's faces in any of his dream states, but at the end he does. So maybe Christopher Nolan wanted to keep things ambiguous? Either that, or Cobb really has succumbed to the perils of dreaming.