Sorry to post and disappear. I've not been able to get near the computer for a couple days. I have actually spent some of that time trying to more clearly define my terms, but I think Nutty started to do it for me:
it's more profitable to try to mark where the idea books started to discover they too had to make at least a passing nod toward the standard signifiers of "good book" quality to make it in the marketplace.
I guess I'm seeing more of a continuum than an either/or (although I fell into the dualism trap at first and thought I was thinking either/or). Everything that follows is IMHO, standard disclaimers apply: Science fiction initially involved authors exploring an idea and the effects of something (a discovery, a political system, a situation). Sometimes those effects had to be played out on characters, but the characters were not the inciting reason for the story. The author didn't say "I'm going to tell a story about this woman who is a psychic and becomes the princess of a planet and loses her husband to war and then has to raise their children to become rulers themselves." Instead, the author said, "What if all the gods ever worshipped really existed because of that worship, and as the worship fades, so do the gods?" It was more speculative fiction than opera.
Thus, The Dispossessed is Le Guin's ideas for how a state might work if perfect socialism could exist, if every person is of equivalent value and even gender has been culturally removed. It's not about the characters although it follows a character so that you can see the effects this has on a person. Le Guin didn't focus on what that would mean for the planet's economic power as a galatic trading entity, or whether people would take vacations there; she was interested in what the effects on people would be. That's an idea, not a character story in my mind.
The Warrior's Apprentice is a character story. It's Bujold telling the life and times of an highly interesting individual, his family and realtionships. She is not trying to analyze the effects of cryogenic life support or cloning on human society, although she does write about them as she tells the story of Miles' life. That's a character-based story, not an idea story in my mind.
I would argue that both Left Hand of Darkness and Ender's Game are idea stories, even though everybody remembers the main characters' names and comes to associate strongly with them during the story. That's just good writing. But the Left Hand of Darkness was Le Guin asking "What if other planets are like Earth in having multiple countries, and what if a more advanced civilization wanted to contact a heretofore uncontacted planet? How would the civilization go about making contact, and how would the nations of the planet react?" Then it slides into metaphor, because she is suggesting that if an advance race contacted Russia first (and this is during the Cold War) the emissary would be held like a state secret...and the same if that emissary had come to the US first.
Ender's Game also came from Card working with the idea of how unthinkingly violent children can be, similar to Lord of the Flies, and how that violence could be made to serve the powers that be. It wasn't about telling the story of this hyper-intelligent third-born kid, although it slid into that somewhat. I argue that this very slide is why people love it. By Speaker for the Dead, though, Ender is no longer a character whose story is to be told, but a mechanism to explore another idea.
The Mars books were a tricky call for me. I think originally Robinson wanted to explore the real science that would go into colonizing Mars, but they turned into a soap opera of who was the child of whom and who was sleeping with who...Gus used the word "dynastic" and that's very applicable here. Also, "dynastic" is an indicator to me of a character-based book.
Dune is another tricky call, and it could go either way. Did Herbert want to write about the idea of a galactic economy based on a very limited resource, and how that would affect power politics? Did he want to write about environmental issues? Religion as a force for political control? Or did he want to tell the story of Paul? I think all of these are true, which is why it's an amazing book.
The Forever War is another idea book, and one that slides into metaphor. Haldeman was writing about how it feels to come back to the US after being in Viet Nam for a year - your subjective experience in jungle combat is intense, and is of a lot more time passing than the time that passed for the average citizen in the US. Space travel was a brilliant parallel. I don't even remember the character's name, but I associate with him keenly even now.
So yeah, I think there's a continuum, and I think books that are about a person or persons and their adventures sell better than books about an idea. They also lend themselves to serializing better. I've been going through the Hugo and Nebula award lists to see if idea books or character books are getting these awards these days, and it still seems like idea books win the sci-fi awards.