Some if it may due to my having read Foucault's Pendulum 5 years before Dan Brown, and part of the plot for Foucault's Pendulum is actually an algorithm for generating Dan Brown-style books. Written about 20 years before Dan Brown's novels. Of course, the fact that Umberto Eco writes much better (and with much greater humor) than Dan Brown didn't help me appreciate the latter.
Some of it may also be due to the fact that I actually love reading on religion, religious history and art history... and that Dan Brown's books very much felt to me like combinations of well-trodden clichés and combinations of barely half-understood history, art, religion, symbolism.
To each their own, I guess.