Hi Damian, all you mention are good (and the bad is bad!).
Making sure that the sellers match up with how the fair is advertised.....is it handmade crafts or a craft and gift fair. This is a particular gripe of mine, attending craft fairs that have quality hand made products next to cheap bought in goods.
The venue is also very important, making sure it is a place people want to come to anyway with the fair being the icing on the cake or if the fair is the primary focus there are other things going on to keep potential customers in the event. Having said all that though, I attend some events which have so much else going on that the craft stalls are a small part and therefore sales are lower, so it is a careful balance.
Making sure that the stall fee is reflective of the amount of potential income for the stall holder and the facilities available.
This one is hard, but trying to make sure that the stall holders are customer focused......not sitting there on phones. If there are still customers wandering around at the finish time that stalls do not start to pack up. After a long day we are all eager to pack up but the customers don't realise that and they come first even at the end of the day.
I'm sure there are lots more.....I may be back!