Stress
You'll have days off, but since you're technically on-call twenty-four hours a day, seven days a week, your "days off" are only off if the animals feel like letting you have them off. Basically, if Bungles the lemur gets a debilitating cough, you're going to have to leave the mall and come in to work.
You'll also deal with the worse parts of the natural cycle of life on a daily basis. Animals that you love will get sick, and some of them won't make it. This is just a part of the deal—the care of these animals is the whole reason you have a job in the first place. If you can't handle watching Bambi or Dumbo, this might not be the right career choice for you.
And have we mentioned that you'll clean up poop at least 200 days out of the year? Talk about work being a mountain of—well, you know.