Cleanliness, good size runs, options for solitary cats who prefer not to mix with others, a good attitude to the cats in their care (which you will observe when you visit), willingness to arrange an inspection prior to booking (or better still for you to call in on short notice, but not at feeding time), no sign of fleas anywhere (booked mine in on short notice and the results were horrible), willingness to feed special diet if required, pref some experience with animals in a professional capacity (e.g. vet nurse or similar), and a nice smell to the place. Mine used to have someone who would take them into their home on rotation for a loaf in front of the fire. Now I have someone live in with them at my place so they are not disrupted in their home. It also saves money as there is a fixed rate for the service and you don't have the aggravation of having to hurtle around two different places for the dog and the cats when going on holiday - much less stressful for all concerned...