A laser focus.
As an entrepreneur, you will experience responsibilities that go well beyond your traditional areas of expertise and interest. You will have to manage all your limited resources (time, money, energy, and people) to get the job done as efficiently as possible. Often times, you will need to let go when things are just good enough because something is more important that demands your resources. That laser focus is only possible if you are able to have discipline, flexibility, and humility. It definitely is not something you know before embarking on the journey but it gets easier everyday once your mindset finally shifts.
I would be happy to share personal experiences and other recommendations having started my own business as well as having worked for some brilliant founders during my career.