Communication. Particularly the ability to cross the boundary between technical talk and non-technical talk.
People like other people who are easy to talk with. Also you will be more likely to build what is actually needed, rather than what someone thought they needed.
One of the most useful books I've ever read was "Thanks for the Feedback" [1]
I've read a lot of the more standard books like "How to Win Friends and Influence People", etc. I feel like this book is similarly valuable, but a lot less well known.
Crucial Conversations was recommended to me on HN, it's also super-useful - on how to talk about difficult/important things, whether in relationships or organizations or everyday life, where most people would be afraid and say nothing. How to speak difficult truths and ensure everyone feels good about it. Not many people would be advanced enough to not get a lot from that book, I think.
People like other people who are easy to talk with. Also you will be more likely to build what is actually needed, rather than what someone thought they needed.