From a commercial standpoint, knowing exactly what your client is looking for and being 100% confident that your vision will give them what they need. Communication between the client and photographer is paramount. It creates a stronger working relationship which in turn, only enhances the final product.
From a personal standpoint, knowing when to click the shutter about a nanosecond before you think you have your shot. Photography has as much to do with how you think and feel as it does with how you see.