In physics you look to great physicists to say who the other great physicists are. Just because Isaac Asimov (at one time) was a best selling, popular physics author doesn't make him a great physicist - in fact, he isn't a physicist at all!
So, surely, it is the expert academics who get to say what is great literature. Who else would you have doing it - Xlktl from literature forum, or a vote from McDonald's customers? Stephen King has only won one prize that is sometimes mistaken for a worthwhile literary award:
http://www.nytimes.com/2003/09/15/bo...phen-king.html
But, overall, he is not at all admired by the literary establishment, so how can he be said to write literature? You might argue that, as King is a modern author, the jury is still out - but the sounds from the jury room don't sound too good for the accused. He might be walking that green mile pretty soon...

