The hardest thing to me is not being able to physically do things while being able to understand them intellectually, understand the emotional arc, having a mental image of what I'd like to do, and realizing what is necessary to do that.
The most rewarding is the blend of the intellectual and emotional in performance. This is not the only art form I do, but in performing music it reaches my whole being with greater regularity, when I'm prone to analysis and the penchant to, as Wordsworth put it, "murder to dissect" in other mediums.
I also feel a sort of awe at times, when my hands are in the midst of some autonomous-feeling choreography, that my body is enacting analogous motions to something a historical figure once did in a completely different context, sometimes centuries ago. It's almost as if a blind mole had stumbled into time travel: suddenly its somatic system, auditory system, and nervous system take on a completely different world, though it remains blind to the vast historical and individual differences. There are interesting historical-perceptual phenomena in other modalities, like seeing an Etruscan wall even though its surroundings or the panorama are so contemporary, but this particular musical one feels more whole and less temporally jarring.