ok, let me entrer into details.
the plot is never the problem. i do not think that the plot of a book is essential. i do not even think that a book must have a plot (or what is traditionally implied by that term).
what i have to reproach to dan brown is his style. outside action, he develops nothing. no character that is well constructed, no general ideas. and what put me off the most in dan brown was the large possibility of making a good analysis of the theme of the feminine being kept hidden in religion. his book was like : fact a lead to fact b which was finished with the action c. i just had the feeling of asking: and then? i did not find in his book anything to challenge me. it was all action from one cover to the other. it was like being on a montagne russe. your heart bumped in your head a couple of time, and that is it. let's go spin in the coffee cups.
thank god they turned it into a movie. it is the best use they could give to the book.
actually, the other nice thing in the success of the da vinci code is the fact that it put in the front an idea which can be considered offensive for the official teachings of the church (did i say this before?
whatever!).