SPOILER ALERT for anyone who reads Alice's page.
Thanks for wanting to introduce me to your Flixster friends, but I think they would have to "friend" me on Flixster in order for that to happen, and that's way too much work. And you know my feelings about Facebook. It's a shock I'm even on Flixster.
I think the "why can't people design their own dreams" question is part of the science fiction aspect of the film. To a certain degree, we have to buy into the idea that these characters have a kind of super-power. Just as we don't ask why can Superman fly, I don't think it's our place to ask why Ellen Page can play God of Dreams. The question about an outsider's control of one's consciousness is far more interesting to Nolan's philosophical overtones than the specifics of "dream magic." The goal of science fiction is to use the extraordinary to garner insight about the ordinary - in this case, the philosophical questions about God and consciousness.
To an extent, I agree with you because I've always had trouble with the fact that movie dreams seem far more cogent and follow a narrative structure unlike real dreams. But to a degree this is accounted for with their "how come you can't remember the beginning of a dream" thing.
I've been thinking about this, and I've decided that the whole movie is Cobb's dream. Why? The kids don't age. They are the same age in Cobb's memory as they are when Cobb meets them, presumably some time later. Cobb's subconscious can imagine himself older, and Mal's subconscious can do the same, but Cobb can't create a future version of his kids. Therefore, I think they're projections - wish fulfillment, like Cobb's eventual heroism and philosophically, like some people's belief in God.
Finally, I had to double-check this: in Greek mythology Ariadne (Ellen Page's character name) leads Theseus out of the labyrinth. So, I think the "clever" name shit doesn't end with "Mal." But once again, I'm willing to forgive the film;