Type of fencing for stall/turnout. Type of waterers. Type of feeders in stall. How much of the stall is sheltered. Amenities such as washrack, farrier area, arena, trails close by, what is paved, what areas have a cover? Extras such as flyspray system or hot water. Are there restrooms there? How often do they clean the stall? Who feeds and cleans, do they tie your horse when they clean, how do they handle your horse when you're not there? Is there a vet/farrier near by or on call? What sort of security measures do they take?
What sort of leniency do they have for late board? Do they have a place to store your tack? Store extra feed?
Is the area perimeter fenced in case a horse gets out? Do they allow stallions? How busy are common areas such as arenas (sometimes there is a trainer that reserves time). How soft are the stalls (lots of pressure sores on the horses in the barn? Do you have to bed the stall yourself or do they provide bedding?). How muddy is it during the rainy season? Do the stalls get flooded? How do they maintain the pee spots in a stall?
What areas are lighted (such as arena) for after-dark visits?
Let me think on it some more. I have boarded my horses all of my life, my parents never had horse property!
Andrea