Ok, I'll start.
The first, most obvious quality would be someone with people skills, who enjoys teaching - some kind of previous experience of teaching would be a benefit in proving that.
The next quality would be someone who is not a natural pilot. The reason is that natural pilots sail through the PPL course with very few problems. Those of us who are not quite so natural struggle through the course, but overcome all the problems. When these people go on to become instructors, they can draw on their own experiences of struggles, problems and solutions, and use those experiences to guide their students through the same problems.
And finally, but least importantly, an enthusiasm for flying.
Note that I haven't included flying skills anywhere in my list of requirements, because I assume that anyone who can get a CPL (or even a PPL, for those who don't want to get paid), an FI rating and a bit of experience already has skills some way above what we need to teach our students to be safe competent PPLs.
FFF
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