Oh, I'm writing this in the moment but it's gonna sit in Drafts for a week or so so folks can get out there and see the movie before I go and play with Spoilery type stuff. In case you haven't figured out what about, read the title again....
Ok, that should be enough time. Spoilers from here on out; bugger off if you don't want to read them.
So, anyways, the Inception ending is quite possibly my least favourite kind of film closing scene yet I can understand why writers love it: done right it offers the chance to add additional layers to what you just watched and make the audience question what happened. Let's consider some of those possibilities:
1) The top was obviously going to fall; it happened off screen!: Awwwww... you cute little kitten... lets take you and your happy ending over here to the kid's table while the adults keep talking...
On a more serious note, this means that the writer, director, and actors have succeeded in one of the harder aspects of filmmaking: seeing to it the characters are likeable enough that you actually give a damn what happens to them and hope for a positive outcome.
2)The top didn't fall; Saito and Cobb are gods of their Subconscious: It's established in the movie that once you're locked at the level Saito and Cobb are at you're essentially gods of your own little pocket universe - is Cobb merely directing Saito in letting them live out more healthy lives there since they've missed all the "kicks" on the other levels? You could easily spin their final dialog in that direction. To flip it around a bit, is Saito backhand delivering on his commitment to Cobb by letting him live out his family reunion in a dream?
3)The top didn't fall; part of it was a dream: Is there a point in the film where Cobb himself is infiltrated? Perhaps when he's alone with the Chemist and Forger? At some later point? Saito does warn him to be more cautious about who he trusts. Was the entire point of the Fischer job not to just implant the idea, but learn how to do it from the one person who would know how? Is his (step?)father using Ariadne to find out what really happened to Mal or force him to come to terms with what he did? These are the kinds of things you layer into a film so that when it goes on to make $200 million+ you've topics to attack in the sequel.
4)The top didn't fall; the entire thing was a dream!: And here's where that suggestion that Psych majors are going to have a field day with this film comes in because, if we accept the rules of the universe as presented, suddenly a good percentage of the characters in the film are aspects of Cobb's subconscious. Who's ego? Id? Creativity? Or, to really fuck with your mind, who's an aspect and who's a possible agent of Mal? Is Mal randomly dropping into his dreams to try and bring him out alongside the Mal that represents his Guilt? What, if any, tells are there? Or, in reverse, has Cobb become so attached to being in the dream state that his subconscious is actively keeping him dream jumping to hide from Mal? Perhaps Mal realized what he'd tried to do to her and locked Cobb in his own dream prison to try and convince him to come back down to where they were Gods again; with only him finally locking her out of his subconscious denying her the ability to control the dream and knock the top off it's axis? Again, threads you leave there to attack in a sequel.
There are likely more options to consider but those are good points of discussion to start out with. The thing is though, this style of ending doesn't work out for the best in every occasion and it's more often "right" to avoid using it. While Inception is a story built around the conceptual argument of what's real and what's a dream - which makes the question posed by the final scene relevant and promotes consideration as to what's really happened - just tacking "---Roll Credits---" at a point of the script which calls for a "conclusive" ending purely for effect frequently comes across as being a cocktease; you want the audience to think something has happened but you won't show them what. Lacking that additional layer of plot to build upon all you're really doing is denying a clear ending to the story.
Which can be incredibly frustrating for the viewer and where a lot of backlash to even something with that depth like Inception will come from.