Communication
...especially about feelings. Communication makes it easier to know what someone else is thinking/feeling/etc- not many people can read minds.

Honesty
if you can't be honest with someone else, chances are you can't be honest with yourself.

Fun > Seriousness
it's important to have fun the majority of the time. if everything is serious and stern, then, uh, what's the point of being in a relationship?

Sense of humor
An ability to laugh at yourself, so you don't get a stick up your ass when your partner is laughing at you :)