The most powerful sign is that your work no longer enchants you - it's not deep, delightful, and mutually satisfying. When this happens, it may be time to look for new challenges.

How fast you are moving is more important than where you are.

If you truly don't have competition, then zoom out until you can define some. Competition can be as simple as the reliance on the status quo, Microsoft (since at some point Microsoft will compete with everyone for everything), or researchers in universities. Pick something, because saying you have no competition at all is a nonstarter.

Inoculate yourself from dangerous bozos.

You need to save some mental, physical, and emotional resources for enhancing your product after you ship. A revolution is a triathlon, not a hundred-yard dash-it requires long distance stamina and multiple skills such as creating, churning, and evangelizing.

Writing a book isn’t an easy process nor is it always enjoyable, but it is one of life’s most satisfying achievements.

At the end of the day in business, it's not about peer review and getting into a scientific journal. You either increase sales, or not.