On a whim last week I reread Pattern Recognition. I was about halfway through Cayce's story when I had an epiphany about Spook Country and Zero History. The thing about PR is that it genuinely is Cayce's story. It's chock full of other stuff about advertising and 9/11, and corporate espionage and the like, but that is all revealed through the lens of a major event of the protagonist's life. ZH has multiple protagonists but they are both involved in the core story of the novel. In contrast, SC's story isn't really the protagonists's story - it is really the story of this offstage character who the reader sees briefly but never really identifies with. Hollis Henry is very passive and seems hardly changed from beginning to end. Milgrim has a significant change, but the nature of his events is that he passively watches the majority of the story and he pretty much drops out of the book just when the real story shows up.
The whole point of Spook Country is really that the movers and shakers of history are often hidden and not recorded. The main story is a shadowy fight between a couple of spooks and our viewpoint characters have no real agency in the matter. That makes it a very passive book. That's why Gibson can take many of the same characters in Zero History and I like the book so much better. Hollis Henry isn't just watching in mild bewilderment in ZH - she's doing things and make shit happen.
So there you have it. After two lengthy book reviews where I sort of hemmed and hawwed my way around what the difference was I can boil it down to that: Spook Country isn't about the protagonists and they don't really undergo a transformation.