Sure, one could simply criticise the science, from the improbable interstellar voyage to the planetology of the titular world, but those aren't the book's main flaw. Instead, it's the fact that it's impossible to engage in any manner at all with the two main narrative characters. Sure, they are a pair of the most obnoxious and unlikable individuals possible - but this isn't normally a problem for a book, as there's often a strange fascination as an author drags you into the heart of evil. Instead, here the characters are immersed in the banality of their misogyny and failure to communicate. With little in the way of characterisation, beyond a basic scene setting, we are left with two men who consistently fail to engage with each other, and the world around them - and yet end up shaping the war that will shake their new home to its very foundations.
There's a sense that Roberts is attempting to write a feminist novel, especially as he tries very hard to use Salt to rewrite Ursula Le Guin's wonderful The Dispossessed. But as he fails to grasp the tao of the journey and the return, and the sufism implicit in the relationships between the statist and the anarchist, Salt leaves you high and dry waiting for a sense of meaning that isn't there. The use of the only major female character in an attempt to provide closure only goes to show how poorly Roberts writes women - as well as leaving the reader wondering if this is a stylistic technique being used to end a story that appears to be going nowhere slowly.
Still, there is something lurking in Roberts writing that bodes well for the future. His descriptions of the arid and desolate landscapes of Salt are powerful, and will remain with the reader for some time. But this is no redemption, and the many flaws in the story still leave you wondering why you even picked up Salt in the first place.