Management. Knowing the financial aspects of a business is key to its success. The people you hire are valuable as well, but I think people skills and the majority of the skills necessary for making hiring decisions will come with experience and age. Finance and supply/inventory skills, however, are more difficult to acquire with experience and are more key to the smooth operation of the business.
One bad employee won't take down a business, but bad money skills will kill you - you can experiment with the former, but the latter is harder to "play around" with.