The attraction to the center seems to produce very erratic pathing - cutting that to zero immediately makes the flocking effect more 'natural'. Then increasing the acceleration factor helps produce those instantaneous changes of direction that are characteristic of animal flocks. With the current defaults the behavior is more 'cloud of flies' than 'flock of starlings' - which is fine, but doesn't show off the large scale co-ordinated movement the boids algorithm can produce. The collision avoidance threshold seems a little high - a slider to reduce that would help make flocks more compact which would produce better behavior when the number of boids is high...