We only hire people who are intellectually rigorous. People who care deeply about finding the truth. People who back up their arguments with data and experiments. People who are intellectually rigorous spend more time asking questions than offering opinions and are excited to be told they are wrong because it means they have learned something new.
We also only hire people who are effective communicators. To build on others' ideas constructively, you have to be able to clearly articulate the flaws in them. This means being able to speak simply and concisely about difficult abstract ideas.
Finally, we hire people who are kind. Having a lot of passionate people with strong ideas occasionally means things can get heated. But it works because we trust each other, we have fun together and we never take things too seriously. No matter how involved our arguments get, we'll be laughing together the next second.
The combination of these three things – intellectual rigor, articulate communication, and kindness, means that we have wonderful arguments and tear through difficult problems.