In YC's case, the number one cause of early death for startups is cofounder blowups.

Every first time founder waits too long, everyone hopes that an employee will turn around. But the right answer is to fire fast.

Don't let the company get distracted or excited about other things. A common mistake is that companies get excited by their own PR.

In general, it's best if you're building something that you yourself need.

Many of the best YC companies have had phenomenally small number of employees for their first year, sometimes none besides the founders.

A winning team feels good and keeps winning. A team that hasn't won in a while gets demotivated and keeps losing.

If someone is choosing between joining McKinsey or your startup it's very unlikely they're going to work out at the startup.