Trust
Love and respect because if you love and respect someone you will be loyal, communicate, and try to understand their feelings even when its hard.
Communication. Boundaries. Compassion.
Granted, I've never been in a relationship, so this is just speculation.
I think the most important thing for both people to be vulnerable to each other.
Often times we build up walls, we lie, and we try to put on a face because (for one reason or another) we don't want to show what we are on the inside. Most of us are scared that if we show ourselves we'll get hurt, and that ends up making everything worse.
If both people can show their true selves to each other then that shows trust, honesty and a willingness to work on the relationship (all the best qualities combined.)
Resonance
Clear cut communication. Being able to express anything and everything is something I really miss from my relationship. No lies, pretending, exaggeration...just saying how you feel.
Communication is key in any relationship. It's important to be honest with each other and set boundaries, as to what you do/don't want out of the relationship. Keeping in mind that you can tell the other person anything is so good :)
Honesty
Trust and commitment, loyalty.
Transparency.
(Being open and honest with thoughts and feelings in a kind way)