What is the one thing that is key to relationships?

communication is key in any type of relationship. With a broken line of communication nothing else works whether it is a marriage or a chain of command.

Selfless love. Being willing to subordinate your own desires to the needs of your partner.
A practical example of this is servant leadership which is a wonderful way to demonstrate love to your partner.

For me is has been being strong friends with that individual and spending the right amount of time each day communicating about everything in that relationship (work, school, children, world events, hobbies, upcoming events, etc...) A nice walk everyday for about an hour is just what the doctor ordered!