The Da Vinci Code, glorified fanfic
The Da Vinci Code is one of the most annoying books movie pitches I have ever read. The (stupendously flat) characters bore each other, themselves and the poor unfortunate readers to death with their endless expositioning. It's a sweet 'n' lo version of Foucault's Pendulum.
Dan Brown's writing reminds me of PG-13 fanfic and children's literature. The hero of the story, a poor man's Indiana Jones, is supposed to be a Harvard professor. He quotes from a copy of 'My First Symbology' with the depth of Keanu Reeves in Bill and Ted's Excellent Adventure and runs around Paris with his very own Mary-Sue-type heroine encountering comic book villains left and right.
Whodunnit? By the end of the story I couldn't care less. Dan, seriously, if you want to write fanfic, at least make it a hard R and let your heroes shag already.
That'll teach me picking up a book with a cover that says "The Number One New York Times Bestseller".

The history stuff I didn't mind but it was pretty bland. But supposedly Ron Howard is to direct the movie version and Russell Crowe is to play the hero.
Granted: the main characters are rather 2-dimensional. And compared to Ecco, Brwon's dialogues are a bit childish. Nevertheless, I find it hard to put the book down and not read the next page.
Beautifully put. I've tried explaining to a few people why I found that book so intensely disappointing. Aside from the weak, commercial thriller aspects of it, I thought the writing was really slack and was surprised that some of those sentences got past an editor.