Even the successful poor writers usually have something going for them. Dan Brown is successful, I believe, because he uses very simple prose and patronizing monologues masquerading as dialogues to fill in required background material - this expands his market to a lot of people who don't read a lot and have a hard time with more sophisticated styles - and his timing of plot points and chapter pacing is excellent.

It's his pacing that I noticed most: it really is first class. But the actual writing is very uninteresting, reminding one of juvenile books targeted at 10 year olds.