Yeah, that's the feeling I've gotten with all his books. Maybe he is aesthetically opposed to active endings. GD did have an ending of sorts, in the historical part of the story, but not in the other-world part--and it seemed to building up to something, so clearly. The ending apparently didn't bother all readers, however. (http://www.guardian.co.uk/books/2009/aug/15/galileos-dream-kim-stanley-robinson)

YRS was a really good book in spite of that, but he also used two speculative premises, not just one, and weaving them together made it all very difficult to manage, I'm sure.