Perhaps the most important aspect of healthy relationships is honesty. Love cannot thrive in dishonesty. It's important to be honest with your partner about everything, no matter how painful or embarrassing.
Ideally, trust and honesty go hand-in-hand. In a healthy partnership, you should be able to believe that your loved one has your best interests at heart at all times.
A sexually "open" relationship is not typically healthy for most people, although some-like minded individuals have managed to successfully finagle such a concept.
A Need for Balance
To be truly healthy, a relationship must fill the needs of both parties. If only one person is being satisfied, the relationship is unbalanced and needs a check-up. Although no partnership is ever perfect, healthy relationships will make both members happy more often than not.