I got through about 2/3 of Red Mars and put it down a few years ago. Even though it was fascinating and unbelievably well researched, it just felt like work to read it. A few years later I decided to try again and ended up reading the whole trilogy. And loved it. I think you have to adjust your expectations a bit and look at his writing for what it is; an exploration on "what would it really be like if X happened".
Most sci-fi is a narrative set in some kind of technologically-different universe. His is more like a serious meditation on what that universe would be like if it really did exist.
Red Mars has kind of a weird rhythm to it. I think Robinson was trying to make a sort of "fake history", so plot lines and characters kinda peter-out or go no where in a way that happens in real life, but not in most fiction (GRR Martin is the only other author I've read with a similar style). I really like the technique, but I can see why its not everyones thing.
The later books in the series slowly take on a more traditional structure, with focus more on a smaller cast of "heroes" about whom most of the action takes place.
It's not the structure of the narrative. It's the writing style itself. Feels very detached and cold, very clinical. The characters have the emotional personality of shadows on a wall; they may have guts but have no heart. When people were dreaming in the 1960s about computers writing literature, this was the stereotype of computer-written text.
Meandering plot lines I'm fine with. I kinda like it in fact.
That's an interesting critique that is very opposed to my own reading. Do you have specific examples? From my perspective, things like the Sax/Anne debate about terraforming Mars, or Frank's resentment at living in Boone's shadow, or Boone's amusement at/exploitation of being a larger-than-life figure struck me as very well developed emotionally.
Most sci-fi is a narrative set in some kind of technologically-different universe. His is more like a serious meditation on what that universe would be like if it really did exist.