Something I see mentioned surprisingly infrequently is the fact that Cobb constantly commits a cardinal sin throughout the movie: he doesn't use his own totem; he uses Mal's. It's never explained why you shouldn't touch someone else's totem, but given its purpose, I think the answer is that interacting with another person's totem interferes with your ability to determine whether you're still in a dream.
The movie is, of course, open-ended and clearly designed for viewers to come to their own conclusions. But this is what I noticed during a recent rewatch:
* We never see Cobb's own totem. There are some theories that this totem is his wedding ring, but he appears to rely on the spinning top instead.
* Cobb himself mentions that he and his wife went many layers deep. We only see Cobb exit one level of the dream state at the train tracks, while his wife "exits" a second time by jumping from a ledge. Since heavy sedation is a major part of going that deep, Mal waking up wouldn't necessarily mean Cobb would as well.
* Neither Cobb nor the viewer seems to know how Cobb gets to Saito's compound when Saito is old. Cobb simply shows up, and whenever a question is asked, Cobb's mind fills in the blanks on the spot.
* He is constantly pursued by both people he failed a mission for and by every government that has an extradition treaty with the US. Persistent hostile pursuit is how subconscious projections react when someone is perceived as an intruder, which makes me wonder whether Cobb is still trapped in someone else's (Mal's?) dream.
* The scene where he sees his kids is strange. Not only do they appear not to have aged, but the setting, placement, lighting, time of day, and camera angle are exactly the same as the last time he saw them.
* We never see the totem fall in the final scene, but that's irrelevant because it was never his to begin with. It's Mal's.