I would say flexibility. It's fairly simple mechanics - if you are more flexible, you can exert more power as the force you put in is over a longer distance (work done (power) = force x distance). I guess the equation puts the same emphasis on both strength (force) and flexibility (distance), but it's much faster, initially, to build flexibility. Just a few simple stretches a day makes a massive difference to your riding, whatever level you are at (if you aren't already a stretchy beast anyway!).