Perhaps what stands out most about Red Mars is the sheer attention to detail. Robinson obviously went to great length to make his account of the colonization of Mars believable from a technical standpoint. Is it entirely accurate science? I don't know, and frankly some of it was above my head. This is probably going to be where this book succeeds or fails for most people. Admire the level of technical detail and consider it a shining feature of hard SF? This may be the book for you. Prefer the softer side of SF? You may have trouble with this one.
But unlike some concept-trumps-all SF, there is indeed a compelling story here. For a certain definition of compelling, at least. It is, essentially, an account of the personalities and conflicts that drive the early colonization of Mars. Future history. And I love reading history. This allows for characters that are interesting, but not exactly leap-off-the-page fully realized. I'm fine with that, since they read like real people who might really do the things that they're written to do.
Robinson also made me look at an SF staple in a new light. He makes terraforming the planet of Mars a hot topic, with major characters on both sides of the issue. Normally, I wouldn't have thought much either way about completely changing the geography, climate, and even atmosphere of another planet, but Robinson made me think about it the same way I would about attempts to do the same on Earth. I wouldn't be an enthusiastic supporter of a plan to turn the Amazon into grassland, so why should I so calmly accept turning Mars into a balmy agricultural paradise? But I had never thought about it that way, until reading this book. And isn't it one of the jobs of good SF to make you think?
If I had access to half stars here, I probably would have given Red Mars 3.5 stars. I'm bumping it up because of the sheer amount of careful thought Robinson put in here, and because I'm always appreciative of SF that makes me think about things in a new way.