This was a wonderful book to read. As I'm writing this review I'm debating on whether to give it 4 or 4-1/2 stars. Although written in the vein of Heinlein's juvenile series, it's obvious it was written by an adult writing about people in their early 20's and many of the themes touched upon should be universal throughout life: friendship, integrity, compassion, so that should not subtract from it's worth. However, the book was a tad too drawn out, the government obviously oppressive, and the characters just a bit too idealistic, so 4 stars it is, which of course still makes it highly recommendable.
This book is probably not what you expect it to be. On my paperback cover is this very science fiction-y view of what looks like a giant aircraft carrier superstructure on what could be a space station with a huge flat plaza underneath and lines of monochrome people giving a sense of `something's' going to happen. And the title has a sci-fi tone to it. So it's with surprise that much of the story is around a protagonist that can best be described as a trobador, on a culture based on romanticized medieval Europe where grace, style, and honor are paramount. Disregard the few references that they are on other planets and a student of Medieval Literature could find this just as enjoyable as a science fiction reader. However, that should not dissuade science fiction readers from reading this. This book had me laughing out loud on almost every page from it's tongue in cheek humor, which can be best explained IMHO as similar to Terry Pratchett or Hitchhiker's Guide to the Galaxy.
The title, A Million Open Doors, refers to the human civilization in the novel that has a Thousand Cultures (not all of which are on separate planets) and an instant teleportation device is discovered called the `springer', and if each springer is considered as a door and if each of the thousand cultures has a door to the other thousand cultures then you have a million doors. Philosophically, Barnes makes a counterpoint to the theme in Dan Simmons first two novels of his Hyperion cantos. There Simmons is stating that instant teleportation is ruining the distinct cultures and/or ecosystems of individual locations. In this novel, Barnes is saying that communication, and because of vast distances between planets even going the speed light isn't sufficient, between cultures is occasionally needed to prevent a culture from going too extreme, as what happens in this novel. And instant teleportation, in addition to allowing cultures to learn tolerance of other cultures, then allows humanity to hopefully form a united front in case of first contact with a sentient alien species.
The protagonist leaves his medieval European culture and is transported to Caledony, an oppressive culture with a cold, raw climate. Even though this culture is described as repressively Christian and capitalistic because of the necessity of monetary tips for services, it really can be considered as similar to the culture of the Soviet Union, where favors replaced tips (I'll trade you hard-to-get chocolate for a lift in your automobile/trakcar/'cat'), and based, instead of on Christianity, on the Soviet System. Barnes may well have been following events of the dissolution of the Soviet Union that occurred in August 1991 when he wrote this book.
One more thing about the protagonist, Barnes makes him realistic in his attitudes from a culture that he is fully representative of to transportation and full immersion into a very different culture. The protagonist realizes there are a quite a few redeemable characteristics of the new culture he first considered bland, and that there are aspects of his previous culture that are somewhat reprehensible, and that as he tries to blend the best aspects of both cultures, he doesn't lose sense of where he came from.