It is important that two people be similar enough that they can make the compromises necessary to get along without either feeling very compromised, yet be different enough to be interesting to each other, to present some challenges, to help each other discover some new things.

A liberal world outlook (you know, what religious people call "love for your fellow man"--the teachings of Jesus, Gandhi, Mohammed--you do not want others to starve so that you can have three SUVs), yet an independence of mind, and from the dogmatism of organized religion, an appreciation of nature, a respect for art, literature, science (you know, the products of the minds of very smart or creative others), and a modesty of self all seem important.

Being on the same side is essential.

Something that enables us to talk, in a relaxed way, not too noisy, not too distracting, so that we can get to know each other.
A walk outside, a visit to a museum, a coffee or a glass of wine....