As an experimentalist who worked in the field for a long time, my answer to the title is: The Equivalence Principle. Because gravitational charge and inertial

As an experimentalist who worked in the field for a long time, my answer to the title is: The Equivalence Principle. Because gravitational charge and inertial mass are seemingly inseparable from one another, gravity is special. It is the concept that makes General Relativity possible and the empirical fact that makes gravitational experiments difficult and unlike any other.