The actual plot is pretty straightforward - there's an Evil Boss and an Evil Acquaintance who both have to be dealt with in fairly unsurprising fashion - but the success of the book is Moon's depiction of what it might feel like to be autistic and write down how your life seems to you. Of course, one must take this as it is - Moon is not autistic, and so this is a literary experiment inviting the reader to inhabit the author's impression of the uncanny valley of autistic experience rather than a clinical description of a real-life individual - but sf fans are used to authors asking them to inhabit imaginary worlds, imaginary cultures and imaginary states of consciousness, so it's not surprising that a book that pulls this off well, as Speed of Dark does, would appeal to readers of the genre.
On first reading, I felt that the ending of the book, when we discover what choice Lou makes with regard to the cure, somewhat undermined the rest of the book. On re-reading, I felt rather more comfortable with it: it seemed to me this time that the climax is signalled decently far ahead and that in fact Moon avoids the temptation of giving the book too pat an ending. There is an interesting use of the New Testament in reaching the conclusion, though I will grumble about inaccuracy in a comment to this entry.
Speed of Dark won the 2004 Nebula Award for Best Novel. The others on the shortlist were Lois McMaster Bujold's Diplomatic Immunity, Carol Emshwiller's The Mount, Kathleen Ann Goonan's Light Music, Nalo Hopkinson's The Salt Roads and Jack McDevitt's Chindi. The only one I have read is the Bujold and I will admit that it is minor; I don't remember hearing much about the others at the time let alone since. None of the shortlisted books was on the Hugo ballot, which is unusual and can sometimes mean a healthy strength and diversity and sometimes mean that the selection process generated a weak list. However, Speed of Dark is a worthy winner.