From email to my sister, who had also just finished it:
I don't read detective novels much (the last one I can remember is Don DeLillo's Libra about JFK's assassination (chock full of conspiracy theory for you) and before that the odd Agatha Christy or Where Are the Children?) so the pacing interfered with my reading in a way I am not accustomed to: Here's something happening! Quick, change the scene to leave your reader tensed and stressful!
In the opening paragraphs, a silhouette stares. That doesn't work for me.
Plus I saw [a spoiler] from miles off and I don't think Brown wrote the [mask the spoiler] scene nearly well enough to have pulled off that conceit. Also I think Brown watched "Eyes Wide Shut" too many times (i.e., once all the way through) and also followed "Indiana Jones and the Last Crusade" too closely. Also "Dogma." I did like how he took little bits of this literature and that history and wove them all together in a plausible way.
Except that some bits weren't plausible: claiming that Tom Cruise witnessed a goddess celebration instead of an orgy in "EWS" means that Brown is delusional, and putting in the bit about the dust spelling out "sex" over Simba's head--which is factual--to bolster a hypothesis about Walt Disney who had been dead for 30 years when "The Lion King" came out turned me off hard. Spoilers: A daughter carrying on two familial lines of a patrilineal society? Assuming Jesus was married because all right and proper Jewish men of his time were married might make sense; but he did tell his disciples (in canonical gospel) that they had to leave their families to follow him; hence it doesn't make me suspicious that his own spouse wouldn't be mentioned. Also if he was a right and proper Jewish dude and married, why ever would his wife be pregnant with their first child when he was at the advanced age of 33? So the implausible detracted from the credible.
The da Vinci stuff, the solving of the puzzles and all, the weaving of known and possible, that was all good too. Except I thought it was cheesy and manipulative of Brown first to give all four lines of a puzzle verse but then with later verses offer them to his readers only one line at a time.
(Someone told me an art historian of her acquaintance gets angry at the title, since da Vinci was known as Leonardo in his own lifetime. The plot doesn't support that particular title, but the market does; and the plot doesn't claim that he was so known, unless I missed that?)