The only Neal Stephenson novel I had read before was The Diamond Age, which I quite liked, although I found it more "hard sci-fi" than I sometimes enjoy. Snow Crash is similar in that it's very much an idea book, but it's also a pretty seat-of-your-pants almost-cyber thriller at times, which is fun.
The basic idea goes something like this: once upon a time, before humans were able to think consciously, we survived by information passed around rather like viruses. Ideas and abilities were contained in these "programs," but humans did not invent more of them, nor move beyond them, for a long time.
Eventually, we did, and the language of these unconscious viral programs died out. We think rationally, speak rationally, and choose our own actions. But what would happen if someone discovered that old, dead programming language, and created a useful tool or weapon out of it?
Who could they control... and who could stop them?
The plot itself I found intriguing--an amalgam of history and linguistics with some odd branch of neurology or psychology, melted together in a way that defied easy separation of fact from fiction.
The characters, particularly Hiro the sword-wielding, wise-cracking hacker, and YT the teenage street surfer, carry the abstraction of the idea and the weight of the plot on a wave of humor, determination, and fear. I really enjoyed both of them... and the Rat Thing, of course.
The writing was another high point. Even scenes that would often be difficult to craft are handled well--YT's, um, encounter with Raven, for example.
I had only two major problems:
1) There's a character who is set up as a deeply devout Catholic (she's also incredibly smart and bad-ass, so I loved her), and then later she's given this paragraph of dialogue in which she casually opines that Christianity is about rationality, rather than the historical death and resurrection of Jesus--that, in fact, one cannot be a "real Christian" and believe in the resurrection.
If Stephenson wanted to talk about that, I'm cool with it... but it's a throw-away moment, an attempt to meld Christianity with what he's doing in this novel but without having to do any arguing or thoughtful explaining. Not cool, sir.
2) The final round-up of the information that Hiro has been gathering is given to the reader near the climax of the story... in a 20-page infodump. I have never seen anything like this outside of the poorer types of fan fiction. *headdesk*
Overall, engaging and enjoyable. I hope Stephenson has grown out of a couple of bad habits in later books, but this did make me want to read them regardless.