I went to a talk by a UK literary agent last week- the subject was "How to get published and make money."
He made a couple of interesting points. Most importantly, only about 30% of adults in the Uk read books ever and only about 5% are "regular readers"; those who read more than 2 books per month.
There are of course, surprise successes in the literary fiction category, albeit these are mainly in the "accessible literary" category- jonathan Coe, Ian McEwan, Nick Hornby, etc but what really sells is fairly formulaic, genre fiction- crime and romances, because this is what the 25% of adults who aren't regular readers, but do read sometimes, buy. The critical factor apparently is that good must always prevail- no-one wants Cormac McCarthy bringing them down on holiday 
, the pace must be compelling and you must create one character with whom the reader will side and root for (no flawed protagonists please!)
That explains why chick lit (shagging and shopping) and Martina Cole (stabbing and shagging), jeffrey Archer etc are so popular.
However, his other point is that it's too easy to get on your literary high horse and forget what fiction really is- story telling- and what many of these fairly low brow writers do really well, even if they don't employ words of more than 3 syllables, is tell stories. They are pace Gods, and that's what most people really want.