Energy... actually I think energy is the most important crosscutting concept across all of science. Of course I'm a chemistry girl, maybe that has something to do with it. Energy is the single thing that connects everything: matter, conservation laws, life, atoms, chemical bonds & reactions. Energy is really hard for students to grasp because of it's many different forms and the way it moves throughout the Earth and the rest of the cosmos. Gotta love it!
Stability and change are not listed in the written material, but I feel it also works in quite a bit. In reading the CCC's, I feel that they are inter-linked and related to one another, so it would be could teaching technique to ask my students questions that require them to articulate the concepts that a unit or lesson has addressed.