High moisture content and long, proper aging. I have noticed a fundamental change in the texture of many cheddars happens anywhere from two weeks to two months after you start aging, and if this change doesn't occur it won't melt. I think it is (from poking around the U of Guelph site today) the curd proteins breaking down, due to enzymes either created or encouraged (it doesn't say, I'm guessing) by the activity of your culture.