Being able to put down a problem or your solution in a manner that the other person can resonate to it; is both hard and important.

If the other person doesn't understand what you are talking about, there's a good chance they won't use what you've built.

This is one of the most important ones and also the one I'm yet to master. I keep making the same mistake of getting excited about something I built and forget that for others to use, they need to understand why it was built.