Okavango,
Some of our best instructors have been those who have come out of our FI course straight from their commercial training.
I have found the main difference is attitude. Not towards flying but towards dealing with people.
Those who have the humility to ask those with more experience (even from non-FI's), and who have a genuine interest in the customer and the school tend to develop quickly to good team members.
As GA is a tight-knit community in Scotland with airline pilots frequenting our schools and clubs, it is no surprise that the more dynamic instructors are earmarked and move on quickly.
While it is not the best paid position in aviation, it is certainly a progressive step towards the airlines. AND, for those willing to put the time in when the weather permits the hours and the pounds do mount up!