Dan Brown's novels are phenomenally successful, so naturally a lot of people have been quick to point out all of their flaws. I thought it would be more productive to examine a few things he does right so that perhaps more people can write successful books.
Cliffhangers This is the main one. You can slate Brown all you like, but he knows how to keep his readers in suspense. Almost every chapter is very short, and ends on a cliffhanger. That makes for about a hundred cliffhangers per novel. That's difficult. If you want to say he's got no talent, try writing a story with that many cliffhangers in it. It's not as easy as you might imagine. The theory of how to do it is articulated well by Jack M. Bickham in his book Scene & Structure, but knowing how to do it is much easier than actually putting it into practice.
Intertwining stories One cliffhanger at a time is good, but Brown manages to juggle several at once. This is achieved by telling several intertwined stories within each book. Scenes alternate between what the hero is up to in his story, what the heroine is up to in hers, and what she got up to in the past. The three stories are told at the same time, each one leaving its protagonist in a precarious predicament, or about to uncover a fascinating insight, while the reader is forced to concentrate on the other two stories for a few scenes. This leaves the reader continually worried or intrigued about what will happen next to the characters in three separate situations.
Real time Each story is set in real time. The narrator never skips large periods of time, telling you what happened "one week later," for instance. You always find out what happened right after the last event. This is a necessity due to the cliffhangers needing to be resolved, but even if it wasn't, it would still have been a wise move. You feel like there isn't a second to spare in a race against time.
A strong female protagonist While Brown's characters aren't perfect, and his books aren't exactly a shining example for feminism, in his favour you have to admit that he generally has one male protagonist and one female protagonist. They have their sexual tension, of course, which adds to the list of things you're wondering about. More importantly, pretty much every reader can associate — at least as far as gender goes — with one of the two main characters. I know this isn't perfect, but if you compare it to how women are represented in, say, the Indiana Jones films, you can see that popular fiction has come a long way. Brown's heroines aren't helpless victims, or struggling sidekicks. They're scientists and cryptographers, and they're just as knowledgeable and resourceful as their male counterparts. The heroines' lives are made all the harder for having to solve their halves of the quests while also having to deal with some old fashioned male bureaucrat who doesn't want them wearing shorts or joining the police force.
Secrets Everyone likes to feel like they're being let in on some kind of hidden knowledge. It's fascinating to find out about things that you're not supposed to know about, so much the better if they're lesser known facts about historic events. Brown's not exactly the best researcher, but he's very good at making the reader feel as if she's getting some kind of insider knowledge. He's good at explaining things, and if nothing else, you'll come away with maybe a little bit of knowledge about the Web's history or the importance of phi as well as having been entertained.
Controversy If you're going to spend a lot of time writing a story, it might as well be about something controversial, that people from both sides of the debate will want to read about. It can be almost as fun, in a weird sort of way, to get annoyed by someone's oppositional point of view as it is to agree with someone. If you debate well, it can even help both sides articulate their arguments. Finding such a topic, such as privacy versus security, or science versus religion, and representing both sides of the argument at least fairly intelligently, is a great way of getting people talking about these interesting issues, and of getting them interested in your story.
Dan Brown has his flaws. Although I'm not up on the history of any religions, I know enough about computers and cryptography to know the difference between a bit and a character, so I know his research isn't the best. The hero is always a blatant idealised version of himself, which is kind of jarring. He's just as bad as Neal Stephenson at putting in reams of exposition. None of that's particularly important, however. In terms of the stories themselves, you can't fault his ability to spin a good yarn, always keeping the reader wanting more. At the end of the day, that's his job, and I think he's earned his success.
If any aspiring writers want to do a better job of what Brown does badly, then that's a good idea, but they should also pay attention to what he does well. They can probably learn something.