I think if you reduce all climbing to its basest elements, you'll find that body position is the prevailing factor.
Balance can be important on a route, but only because having the right balance or the right points of contact allows proper body position. Finding balance literally IS body position.
Distance is a function of difficulty on a route, but only because it means the climber must create a change in body position to find success. You can see this every time a novice climber "slaps" at a hold they can't reach, or a more adept climber turns their shoulder out and shoves their hip in the wall on a big lockoff.
Bad holds are frequently thought of as a good barometer of difficulty. But often body positions with better feet, different hip turnout, or more relaxed stances can make bad holds feel "restable." A good example of this is sloping crimps. On an overhang, sloped holds without the use of toe or heel hooks usually means keeping the feet lower will increase traction on the holds. When the climber goes to move their feet up, the holds become more difficult to hold on to - because of a change in body position.
Footholds are also used as a route's yardstick. Our local bouldering area has a lot of granite pebble smearing, which requires the foot to stay at a certain angle. This angle means the leg stays pointed in towards the wall, and your butt stays pointed out. If you pull your hips in too close, you might gain distance for a move, but the body position will compromise foot traction and cause you to fall. Again, body position is the building block of movement. If I were going to try to duplicate one of these routes, I know I'd need to keep the feet pretty awful. Solid feet that you can edge on allow the hips to come in farther, the heel to be raised further, and in general just allow the climber to do more.
It is functional to set routes that force movement based on balance, distance, hold type, etc. But I think setting by forcing moves that require specific body position is the most effective way to really make a climber solve a puzzle.
Applying this to routesetting is difficult. It's one of the things I try to think about most when I'm setting. If a route feels "classic," it's usually because the body position shifts a) feel natural, b) are not dramatically harder from one to the next and c) are less body-dependent.
I'll talk more about body position in a future post. For now, happy setting!