The Criteria seems to change from year to year...I've only ever liked one winner in the entire history of the competition.
Personally I think it was a mistake to call it the Turner prize as not one person who has been nominated or won in the event has even one tenth the skills of Turner plus he can hardly be the epitome of contemporary art of the last 60 years..it should have been named after a British artist whom was rather more contemporary like Francis Bacon,Henry Moore ,Barbara Hepworth,Bridget Riley,Peter Blake,David Hockney and my particular favourite Stanley Spencer...
Personally I think it's not so much a case of Contemporary art more like Contentious Art.