Why is The Da Vinci code so bad?

I haven't read Dan Brown's potboiler - my wife's reaction was so vituperative that it put me off. Life's too short, surely? I am intrigued, however, after seeing Tim Footman's new blog which sets out to analyse The Da Vinci code chapter by chapter and determine "why a book that ... is so egregiously badly written, is so successful".

I haven't read much of it yet but I can tell it'll be fun simply from Tim's post about the title: "So, as a title, The Da Vinci Code screams either "I have no idea what I'm talking about!" or "I have a pretty low opinion of the intellect of my potential readers" and I'm not sure which is worse."

I'm tempted to get a copy of the book and join in. I could probably use an emergency book for 26 books at some point.