Okay mine is rather detailed..
First and foremost, horses on the property must not be starving.. my horse is a hard keeper and I won't even go into the premises if the horses are looking poorly kept.
Second.. and the most important. Professionalism from the managers and staff! They also need to know what they are doing, and CARE about their job and the care of the horses, including mine and the minis no one rides or the retirees, at the barn. If this requirement isn't met, I will not board there.
Third, an indoor arena is so important because I want to be able to train my horse in any and all conditions (except no lightning in a metal arena :P) so that he will be as fit and happy as he can be.
Fourth, it has to be full care.. and GOOD QUALITY full care. Meaning good quality hay, checking him all over for kicks or cuts or other things. He needs to have his stall cleaned so he isn't standing in filth (and if he is, I should be allowed to clean it) I've tried field board and he just looks awful.. but I may try it again since he was sharing with a LOT of horses.
Fifth.. daily turn out. Or my favorite.. turn out 24/7 unless it is below freezing or they come in to eat (love my current boarding situation!)
Sixth, locked tack room.. my saddle fits my shark withered horse to a T and if someone stole it... oh my I would not be a happy camper! Along with my most favourite brushes that were so hard to come by (I am a very picky person...)
Trails... love them
Hills, for hill work of course
Hot/cold water, because there are times my horse needs a bath (rainrot, unhealthy coat, etc.) during the winter. Wash stalls are nice, but I just need a hose and a reserve.
Free choice hay.. love it because my horse requires a lot of hay to keep his weight (and a lot of grain.. but I'd supply that)
Willing to blanket.. I just started turning my horse out with a sheet because I want him to retain the weight, not shiver it off because he is cold and wet. It's working so far :)
Friendly boarders.. I love people that ask if I want to ride with them or go to a show or whatnot.
Allow outside trainers... because sometimes the right trainer does not work at the barn you are looking at! Mine right now is so amazing and she drives a good 40 minutes to instruct me and a friend. The ones at my barn are nice people but not what I'm looking for , for my horse.
Farm call vet and farrier days... they are amazing! It's nice to have access to a fabulous vet and farrier (though you can call your own in of course) without having to pay for mileage.
And the best thing about this list? My current boarding place meets them all :)