And I would rather have seen a movie with Truman's story compressed into the first 15 minutes, and the main part of the movie being about his learning to live in the larger world.

I disagree. As I see it, The Truman Show is, at its core, a Gnostic parable similar to The Matrix, but better executed. It follows the protagonist's journey of discovery, as he begins to get hints about the true nature of reality; namely, that the world he thought of as "real" is, in fact, a prison of illusion. In the end, he is able to break through the illusion, confront its creator, and reject his offer of a comfortable life inside the illusory world, in favor of the much less comfortable yet fully real world outside.

In this parable, the Truman Show dome stands for our current world (which, according to Gnostics, is a corrupt illusion); Christoff stands for the Demiurge; and the real world outside stands for the true world of perfect forms / pure Gnosis / whatever which can only be reached by attaining enlightenment (for lack of a better term). Thus, it makes perfect sense that we don't get to see Truman's adventures in the real world -- they remain hidden from the viewer, just as the true Gnostic world is hidden from us. In order to overcome the illusion, Truman must led go of some of his most cherished beliefs, and with them discard his limitations.

IMO, the interesting thing about The Truman Show is not Truman's adventures, but his journey of discovery and self-discovery. Sure, we know that his world is a TV set, but he doesn't (at first, that is). I think the movie does a very good job of presenting the intellectual and emotional challenges involved in that kind of discovery. Truman isn't some sort of a cliched uber-hero like Neo; instead, he's just an ordinary guy. Letting go of his biases, and his attachments to people who were close to him (or so he thought) involves a great personal cost for Truman -- which, surprisingly, Jim Carrey is actually able to portray quite well.

Sure, it might be fun to watch Truman run around in the real world, blundering into things and having adventures, but IMO it wouldn't be as interesting or thought-provoking -- even accounting for the fact that Gnosticism is, in fact, not very likely to be true.