I think most of the criticism relates to his clumsy writing style, his grammatical errors, sentence structures etc, rather than his story-telling ability. If you compare his writing style to other (great) thriller/crime novels, you'll spot the oddities in Dan Brown's writing. Example below (taken from here
http://www.telegraph....orst-sentences.html#)
The Da Vinci Code, chapter 4: A voice spoke, chillingly close. "Do not move." On his hands and knees, the curator froze, turning his head slowly. Only fifteen feet away, outside the sealed gate, the mountainous silhouette of his attacker stared through the iron bars. He was broad and tall, with ghost-pale skin and thinning white hair. His irises were pink with dark red pupils.
how can someone be chilling close, yet be 25 feet away? How can a silhouette have colour definition?