This was not a sure thing. Even though it's based on the bestselling (and damned good) space opera by James S.A. Corey, The Expanse could have easily screwed the pooch. Syfy has a history of bad, bad, terminally dumb shows, and they could have found a way to make this one of them.
One subtle but important point - the showrunners knew enough to keep the books' vertically built spaceships. In the books, Corey (pseudonymous authors Daniel Abraham and Ty Franck) makes the point that in a future without artificial gravity, spaceships with constant-gravity drives will essentially be skyscrapers sitting on top of thrusters. They won't swoop through space like the Millennium Falcon; they will shoot upward, balanced on their tails. The temptation for any space opera to portray spaceships as super-fighter-jets is enormous; resisting it takes real aesthetic willpower.
So that's all gravy.
But there are already some troubling portents, ominous harbingers of what might be coming. The showrunners were able to resist the siren song of the horizontal space ship, but they couldn't fight the lure of the glamour-puss. Most of the main characters are either prettier, neater or shorter than they should be.
In the books, there are profound and obvious physical differences between Earthers and virtually everyone else: Those born on Earth are shorter (and presumably stronger) than those born elsewhere. Belters especially are tall, thin, and unable to spend time comfortably on Earth. Holden, the main character in the books, is handsome, but he's also Earthborn and considerably shorter than characters like Naomi Nagata and Detective Miller, the other protagonist. The same is true of Havelock, Miller's partner and another Earther. But on the show Steven Strait (who plays Holden) towers over a shorter (and very curvaceous in a distinctly un-Belterly way) Dominique Tipper as Naomi. And surrounded by Belters, Havelock should have a tough time seeing through a crowd. In The Expanse, he doesn't.
Then there's Wes Chatham as Amos Burton. Chatham is ex-military, and believable as such. But Amos Burton *isn't* ex-military. In the books, Burton is deeply damaged, the victim of an abusive childhood who grew up on the streets. He is very big, and very strong, and very scary - but he doesn't look like a youngish poster child for the armed forces; he looks like someone who got beat until he got big enough to beat back. Chatham seems too "together" to be Burton. (To be fair, Chatham might pull it off - he hasn't had much screen time yet; but at first glance, he doesn't project the aura of damaged violence that Burton should have.)
These casting notes might seem like quibbles; they're not. The height question, for instance, goes to the heart of the book's main political conflict: Earth/Mars vs. Belters/OPA. The Belters aren't just terrestrial humans in space; they have changed. They can never go back to Earth. They can barely survive that gravity. They're stuck in the black forever, with their backs to the figurative wall. There's one scene that nods toward this fact, but it's then ignored with every crowd scene on Ceres. Amos and Avasarala are two of the most compelling characters in the books, but casting may have sanitized them.
Still, The Expanse is worth watching so far. Let's see where it goes.