* Build products
* Sell products
* Recruit
* Build rapport with customers
* Evaluate business models
Most of that is ability. Few people have ability in /all/ of those directions. I've seen people who are even competent (not great) at all of those launch successful business after successful business.
By "successful" I don't mean billion dollar unicorn. I mean businesses which employ tens (or hundreds) of people, have millions in revenue, and last indefinitely.
This stuff isn't rocket science. If a Fortune 500 make widgets, and you make higher quality widgets at lower cost and make people aware of that, you'll run a sustainable business.
As a small business, on one hand, you'll have slightly less access to distribution and marketing channels. On the other hand, you'll have a much leaner cost model (no multi-layer executive hierarchies to sustain). Your team will also be much more focused, productive, and agile. You'll also be more risk-tolerant. All of those provide competitive advantages.
I've seen $100mm+ companies fail because Google decided not to like them anymore. I'm talking about external forces that are far more powerful than execution. And there are lots of 'em (an infinity, even!).
1. High-risk venture. Someone wants to be the next Facebook and takes a 1-in-a-100 gamble at a billion dollars.
2. Stupid idea/business model. Founder is a true believer in some idea which doesn't match reality.
3. Poor execution. It's a good idea and business model, but any of the hundred things which can shut down a business do: technology takes 10x as long to develop and is buggy; founder fails to recruit good people; zero external sales/visibility/marketing ("if you build it they will come" or wildly implausible assumptions about viral or similar); costs get out-of-control; etc. Most founders I know can do all of those reasonably, and one or two brilliantly. It's hard for me to emphasize how important (and hard) it is to be at least competent at all of those since messing up any one will shut down a company.