Luck is very important in any startup, I believe it is the major factor in determining startup success.
This may be a demoralizing and repugnant thing to hear for the Type-A "control my own destiny"-style of people who tend to populate Silicon Valley, but I think it's true.
Startups tend to fail fast. Within about 18 months it is usually easy to tell if a startup is going to go somewhere, and it is culturally okay to just give up, close down shop, and try something else. This means it's possible entrepreneurs to roll the dice over and over again until luck hits.
The industry is good at spotting lucky breaks and exploiting them. VCs and other experienced industry veterans have, over time, become pretty good at spotting a success story in the making, and are usually ready to help pour resources (money, people, experience) into a startup that's hit a lucky break. A lucky founder doesn't have to already have a perfect team in place or be the perfect executive - if they have enough of that elusive x-factor (usually it's "will and determination") to grow with the company, the Silicon Valley ecology stands ready to provide them with funding, top-notch talent, and the experience of others.
For what it's worth, my view is that start-up success is driven most by the product passion, quality, vision, team-work and persistence of the founding team and the talent that the team attracts. Breathtakingly brilliant, intellectually curious entrepreneurs who are always thinking about how the rules of the game are changing lead and drive the biggest successes. However, I never discount luck: if it perhaps is not the major factor in success or failure, a certain level of humility combined with luck, good timing and a few fortunate breaks often makes a real difference.
Luck is about being in the right place at the right time when it matters ... and the 99 times when it didn't. Thank you.