First of all, Cryptonomicon, by Neal Stephenson, is a hefty tome that is easily three books, not just in sheer number of pages, but in the number of stories being told. World War II stories of soldiers and mathematicians are mixed with the stories of computer communications development in the late 90s. Serving heavy doses of war stories, cryptology, treasure-hunting, math, computers, special operatives, and much more, Stephenson builds a multi-generational novel from actual history and technology. The fast pacing of the story is occasionally broken up with sections detailing the mathematics or codes being used by characters. And while I was interested in these sections, it's difficult to take the time to really read them while another character, in another place and time, is at the brink of death or in terrible danger(and they always are).
After the epic chronicling of multiple stories, Stephenson's ending leaves much wanting. While ties have been drawn between generations and certain conclusions therefore reached, the novel ends in the middle of heightened action; the characters could die, they could become rich, or all might be forgotten, reclaimed by the jungle. But Stephenson never says, he just abruptly stops on the edge of a cliff, leaving speeding readers to fall off. With the extensive development of characters and the many faucets of their stories, they (and readers) deserve at least a ledge or branch on that sheer face of cliff.
But sometimes the fall is worth it- and in this case Stephenson gives readers so much that they'll manage to construct something before they hit bottom. Or they'll just get another of his books.