Anathem is a prolonged meditation on the essence of civilization, a manual on reasoning, a page-turning adventure story, a hymn to empiricism, and much more.
It starts off squarely in the tradition of Walter M. Miller's A Canticle for Leibowitz, and ends squarely in the tradition of one of those extremely long First Contact novels. The first three hundred pages of Anathem are the best part... (but then this is probably true of most nine-hundred-page novels that actually get published...)
Sometimes when I'm trying to persuade my daughter she'll like something, she expresses her disagreement by singing a song of her own invention that goes “my dad's a nerd, my dad's a nerd...” I'm concerned here not to make Anathem sound more nerdy than it is, but I think it is more nerdy than Cryptonomicon. Stephenson plays with such ideas as that Platonic forms originated in the first few seconds of the big bang (assuming I understood this part correctly) -- and he plays with such ideas using completely made-up terminology specific to the monastic orders in his invented world. That sounds a bit intimidating, but the way Stephenson writes, it really isn't... (this is where, in my head, I hear my daughter singing that song...)
Anathem is also more elitist than Cryptonomicon in its sense of how the world works – Stephenson seems drawn towards the idea that all civilizations will tend over time to become nerdocracies. Late in the book, he introduces the concept of “fascination burnout,” a feeling with which some readers, by that point, will have started to empathize... but overall Anathem is, despite its vast conceptual reach, a fun read, the kind of book Iain M. Banks, Umberto Eco, and Douglas Hofstadter might come up with if you locked them up in a monastery long enough. Stephenson is also (again, assuming I'm following him correctly) the first person ever to make the idea of the brain as a quantum computer strike me as even remotely believable -- I'm not sure how he pulled this off because, now that I've finished Anathem, the idea's gone back to being unbelievable again.
“Orolo nodded. 'Quantum interference – the crosstalk among similar quantum states – knits the different versions of your brain together.'
'You're saying that my consciousness extends across multiple cosmi,' I said. 'That's a pretty wild statement.'
'I'm saying all things do,' Orolo said. 'That comes with the polycosmic interpretation. The only thing exceptional about the brain is that it has found a way to use this.'”