When I used to be the boarder and not the boarding facility. The most important things to me were Safety. It could be the nicest facility ever with the nicest people but if it had a line of barb wire fencing it would be a deal breaker for me. Its important that you accept their feeding program, vet, farrier allowances. I do look at the condition of the horses as a whole... I've gotten in a nasty horse or two and I would hate to be judged on a new boarder!
Also one of my other deal breakers is how clean it is.. The facility could be ugly but if its not clean and functional then I wouldn't board there.