Was wondering what should be backed-up. Interested to get the views of those who have had to reinstall.
It's a good question - I've done it many times.
Basically everything you've fiddled with from bog standard default - try Addon Scenery, Scenery, Aircraft, Gauges, effects etc etc etc., if in doubt save it anyway.
Just ask yourself if I had to go back to default, what would I lose.
Good luck.