Derek Trucks is an absolute savage on guitar. I'm a big metal fan so I'm familiar with plenty of guys who are more technically proficient (Petrucci, Malmsteen, Mustaine etc), but Trucks has an "it" factor that is hard to describe or quantify.
Couldn't agree more. He's my favorite living guitarist, and I think Jerry and Duane are the only ones I would put above him for my tastes.
I describe it as knowing how to use space, especially the space between the major and minor chords, which is something at which I think each of them excels/excelled. From what I understand, it's similar to the modal approach for which Miles and Coltrane are often credited, and it has as much to do with the notes they
don't play as with the ones they do.
I suppose "elegance" captures the essence as well as any word I can think of.