That's the thing about life - you only regret your own mistakes :) Having said that I thought there were several pointers that resonated with me:
* Market is everything - this is the #1 rule. If what you're selling is something that has a large market demand, then pretty much every other issue is nullified. You can screw up and still come out ok if the market is big enough. If you understand your market, the product and its evolution and pricing and marketing all fit together nicely. And the simplest way is to ask for feedback from the market.
* Fit with customer behavior if you can - don't try to make them change behavior or even think. Lower the cognitive friction the customer has to suffer.
* People - treat all people you meet with respect - customers, employees, investors. Everyone.
* Understand the role of luck and that of skill - you could make all the right moves and get a product out there that's awesome, but if say you picked the wrong people, it could all unravel in the blink of an eye. You can control only what you can. Success is often not deserved, nor is failure.