The problem is... now I feel like I have to read something else, quick, to get the taste of Inferno out of my mouth... brain... whatever.
First of all is Brown's writing style, if you want to call it that. He regularly commits a bunch of literary sins that tend to grate on my nerves; just that fact that Robert Langdon, in his own POV chapters, calls himself Langdon instead of Robert drives me up a wall. If you're writing from the POV of a particular character, not an omniscient narrator but actually inside the character's head, then you need to use that character's names for things.
There's also the enormously clunky way Brown writes flashbacks, the ever-growing Marty-Stu-ness of Langdon (eidetic memory! Has been everywhere! Knows everyone! Women love him! Invited to celebrity events! Dude, Dan... the guy's a professor and an author, get a grip), and his irritating tendency to give his characters soap-take dialog.
For example, instead of this:
Langdon: "Oh my God, the statue we're looking for is in Vanuatu!"
...we get this...
Langdon: "Oh my God!"
Langdon: "Why didn't I see it before?" [insert several paragraphs regarding symbols and history and stuff.]
Sidekick: "Robert, sit down and catch your breath!"
Langdon: [sits down, thinks about how he'd like to be sitting down at home in his custom-made leather blah blah blah]
Sidekick: "Now tell me, what have you discovered?"
Langdon: "The mad professor's note said that the statue we have to find is in a country where [blah blah blah]. That means it must be in a country where [blah blah blah]!"
Sidekick: "I don't understand!"
Langdon: "The statue... it's in Vanuatu!"
No, there's nothing in this novel about Vanuatu. I was going to fill in the blah blah blahs with five minutes of research on Vanuatu, but then I got bored.
Let's see, what else. Oh yeah, back to the naming thing. If you have a character who only knows another character by an alias, it's perfectly okay to have them think of that character as that alias. However, if you have a chapter from the POV of the aliased character, it is not okay to have them think of themselves as that alias! "I.C. Weiner looked out the window of the cryogenics lab..." No. No no no. That's just lazy writing.
Also, if the POV character sees something happen to another character and misinterprets what he sees, that's okay. However, if you have a POV chapter from that other character and are intentionally vague and obtuse about what is happening so that it seems to corroborate the first character's misinterpretation, now you're plain being dishonest.
And that's just style. We're not even talking about content. First of all, the bad guy was another one of your boilerplate "ultra-rich genius scientists who delights in concocting convoluted riddles to vex his adversaries" which exist nowhere in reality. So while this genius was busy inventing things that had never been invented, we're also supposed to believe he had tons of extra time to photoshop a painting with a hidden phrase and put it in a weird little cylinder and put it in a safe deposit box and that leads to another painting which leads to a mask which leads to a poem which leads to oh God I don't even care anymore.
Also: AMNESIA. Argh. And fake hospitals and fake assassins and Danny you just make me feel like I can't trust you at all. There's a line between being clever and just screwing with the reader and you have crossed it.
Finally... the climax. The only point at which I wasn't surprised was when they discovered that the bag was gone. At that point you knew it wasn't something like the Black Death, despite the genius' love of plague references. But then they discover what's actually been done to the population of the Earth and it's like...
Oh. Hmm. That was kind of rude. But you know, his heart was in the right place, after all, and maybe it wasn't such a bad idea.
Yeah, I mean... I don't even know what to say about that part.
And obviously I'm part of the problem, having bought the book myself, but if this is the kind of stuff that passes for best-selling these days (along with the likes of 50 Shades of Rewritten Vampire Porn), I'm not sure I want to live on this planet anymore.