Prices and insurance are the top two. Also what you are going to provide. My barn provides buckets, pitch forks, grooming supplies and temporary halter and leads if you loose yours at no extra cost. Most barn ps don't offer this or they make you pay extra.
Insurance, insurance, insurance. Make sure you have the capital to maintain a barn and your horses.
Insurance is a huge one, the condition of the place, the price of the feed, the price of the wood chips/shavings. The amount of work to up keep the place. B*tchy customers, drama (where there's people there's drama), how to advertise...ect.