Lee Child is OK, they're not great literature but at least they're not badly written. Same goes for John Grisham.
Patterson's actual writing is so bad. The dialogue, the descriptions.... the only "trick" he knows is to have very short chapters so the books move fast. I HATE the bits in the Alex Cross books where he's with his kids, his various girlfriends or his grandmother - not only are they tedious soapy distractions from the main plot, but the way the characters interact, the conversations they have make me cringe!! :x
I'm often surprised by how many popular writers are actually very bad at writing! Dick Francis, Clive Cussler, Michael Crichton (Congo is the worst book I've ever read). Critics always take the piss out of Dan Brown's writing, but he's not the worst - I think his first few books were OK, but The Lost Symbol was terrible.