The Postman by David Brin
The answer to the title question is: a lot. For the sake of retaining readers I will refrain from making too many.
So The Postman is about this guy named Gordon who is struggling to survive in post-apocalyptic America. Apparently this is something that is quite difficult to do. We first find Gordon struggling in the mountains of Oregon fighting off a group of bandits. Well, maybe fighting off isn't a good term, perhaps running away from? Yes that works much better. As you can see Gordon is not your typical heroic character. He is an ordinary guy just trying to survive and hoping to find somewhere in the United States where some form of order remains intact instead of the isolated settlements struggling to survive and the roving bands of pillagers that are so common back east.
In his flight from the bandits Gordon discovers an abandoned postal jeep. One thing leads to another and Gordon finds himself posing as a Postal Inspector for the Restored United States Government; all of which he totally made up himself in order to not be killed by some paranoid villagers. Soon Gordon is traveling from town to town bringing letters and setting up post offices all the time struggling with the knowledge of the facade and his longing to find somebody who is restoring order.
Do you see where this is going? Yeah, so did I about 4 chapters in. Gordon on the other hand doesn't figure out that he is that order-restoring somebody until the end of the book. Actually, not even then. He's kind of an idiot, really.
Basically, in my opinion, The Postman fails to deliver (okay, sorry, but honestly Brin used a postal pun in a horribly inappropriate situation so really, this isn't as bad). It seemed like the whole driving force of the book would be Gordon's realization that even though he was lying to all these citizens about a restored government, he was helping to create one himself through the hope that he was providing. Unfortunately, that doesn't sink in. Instead there's some action sequences, fighting, romance, strangely misplaced chauvinism, and more self-doubting than you can handle with a 10-gallon bucket. Through it all Gordon spreads hope and reopens communication lines creating a small pocket of civilization in Oregon without ever realizing the impact he is having.
Only one line on the second to last page even hints at this getting through to Gordon. I'd quote it but I already returned the book to the library. It was something about people believing in their myths so much that they eventually make them true. It was the best line in the book, which, from a stylistic viewpoint, wouldn't have taken much.
I've started reading another Brin novel since I finished this one otherwise I would dismiss him as a scientist posing rather poorly as a writer. Now that I have some more perspective though I think The Postman just wasn't his best stuff, despite the various awards it won. Honestly, I don't understand that aspect.