Nothing. Hills is a bad cat food brand (good dog food though!).
I've typed up this spiel a few times today and I don't have it in me to do it again. in short, though, feline nutrition is a budding science. cats are obligate carnivores, which means they are designed to only process meat product. hills uses rat studies (which are omnivorous) for their nutrient basis. this is great for dogs, which are also omnivorous, but not cats. furthermore, even high protein foods can be bad if they use low quality products, which can result in kidney failure. hills uses low quality protein on top of being carbs heavy.
products I've had great success with that use good protein sources and look towards the archetype feline diet: