Team, Product, Market

Every startup reduces to three core elements. Team: how suited the CEO, senior staff, and key employees are to the opportunity in front of them. Product: how impressive it is to the people who actually use it, judged on features, usability, speed, polish, and bug count. Market: the number of potential customers and how fast that number is growing.

A subtle but important point: product quality and market size are completely different things. You can build the best product ever made for a market that doesn't exist. Great engineers do this all the time.

The interesting question isn't which of the three matters. It's which matters most, because that's where you should spend your worry.