However, having recently read Stephenson's Quicksilver, I can see more of a resemblance to Pynchon. With Quicksilver, Stephenson displays a great deal more playfulness with style and narrative. He's become more willing to cut loose, explore language, fit the style to the situation he is describing and even slip out of "novel" format, whether it be the laundry list compilation style of Royal Society meeting notes (for example, pages 182 - 186), the epistolatory exploits of Eliza (pages 636 - 659 among many others), or theater script format (pages 716 - 729). Stephenson isn't quite as spastic as Pynchon, but the similarities between their styles are more than skin deep. In addition to this playfulness in the narrative style, Stephenson, like Pynchon, associates certain styles with specific characters (most notably the epistolatory style that is used for Eliza). Again, Stephenson is much less radical than Pynchon, and only applies a fraction of the techniques that Pynchon employs in his novel, but Stephenson has progressed nicely in his recent works.
Most of the time, Stephenson is considerably more prosaic than Pynchon, and even when he does branch out stylistically, it is done in service of the story. The Eliza letters again provide a good example. The epistolatory style allows Stephenson to write for a different audience. We know this, and thus Stephenson has a good time messing with us, especially towards the end of the novel where he takes it a step further and shows Eliza's encrypted letters and journal entries as translated by Bonaventure Rossignol (in the form of a letter to Louis XIV). All of this serves to further the plot. Pynchon, on the other hand, is more concerned with playfully exploring the narrative by experimenting with the English language. The plot takes a secondary role to the style, and to a certain extent the style drives the plot (well, that might be a bit of a stretch) and while Pynchon is one of the few who can pull it off, Stephenson's style doesn't really compare. They're two different things, really.
Nate has a great post on this very subject, and he shows that a comparison of Quicksilver with Pynchon's novel Mason & Dixon is more apt:
The style of Mason Dixon is a synthesis of old and new that hews remarkably close to the old. Stephenson, on the other hand, writes in a much more modern style, only occasionally dotting his prose with historical flourishes ... The distinction here is an old one; classical rhetoricians spoke of Asiatic versus Attic style - the former is ornate, lush, and detailed, while the latter is lean, clean, and direct. Stephenson is a master of Attic style - a fact that's often obscured because, while his sentences are direct and elegant, their substance is often convoluted and complex. You can see it more clearly in his nonfiction - look at his explanation of the Metaweb for an excellent example. Pynchon, as an Asiatic writer, will elicit more "oohs" and "ahhs" for the power and grace of his prose, but will tend to lose his readers when he's trying to be florid and tackling difficult material at the same time. Obviously, both authors will tend toward the Attic or the Asiatic at different points, but in general, Stephenson wants his language to transparently convey his message, while Pynchon demands a certain amount of attention for the language itself.I haven't read Mason & Dixon (it's in the queue), but from what I've heard this sounds pretty accurate. Again, he makes the point that Pynchon and Stephenson are on different playing fields, appropriating their styles to serve different purposes... and it shows. Stephenson is a lot more fun to read for someone like me because I prefer storytelling to experimental narrative fiction.
I recently read Pynchon's The Crying of Lot 49, and was shocked by the clarity of the straightforward and yet still vibrant prose. In that respect, I think Stephenson's work might resemble Crying more than the novels discussed in this post...
Update: As I write this, Pynchon is making his appearance on the Simpsons. Coincidence?