In a word, eggs. The more slowly you cook eggs, the more creamy they will be when set. I generally bake my cheesecakes at a fairly low temperature, sometimes as low as 225F or 250F. The upside of this method is you end up with a super smooth and creamy cheesecake that also stays perfectly flat and free of cracks. The downside is it takes a super long time to bake, possibly as long as 3, 4, or even 5 hours depending on how much batter you have. And that is just kind of crazy, unless you have nothing better to do. My goal is to bake at a low enough temperature to keep it nice and creamy but not at such a low temperature that it takes forever to bake.