What would you say is THE MOST important factor to good imaging?

Another vital factor is the quality of your hearing. If the sensitivity and frequency detection of your ears is different, your brain can’t optimally decode and place images in their proper place. I know from personal experience.

"Balance control knob"  

Don't laugh. It's not funny - not when employed properly. 

Stay tuned for my latest review at Dagogo.com, in which I discuss a fascinating speaker system that allows, among other things, for very precise manipulation of the center image via controlling left and right speakers. 

I also had yet another unusual and highly efficacious feature of the Legacy Audio Valor and Wavelet System (Besides L/R centering) that allowed contouring the center image to give the impression of moving the listening chair closer or further from the performance, like an adjustment from "front hall" all the way to "back hall". I sorely miss that feature, because it was compelling!  (Also reviewed for Dagogo.com)

Time and phase accuracy which includes pistonic motion drivers :-)

Contrary to George’s opinion, horns can provide excellent imaging if somewhat reduced SS depth. Just another pointless dig at horns he feels he must insert. Oz

The more directional the better, that’s why esl’s, ribbons, planer ect are so good at imaging, width, depth and placement. But are usually one to two person hot spot seating, horns just have a diffused "wall of sound." 
Iv’e owned LaScala’s, Heresy’s and Forte, they don’t come close to esl’s, ribbons, planer ect.

Cheers George

Well that's that then... Let it be known horns do not image (looking for gavel to hit table)