Communication's the key. Knowing someone only comes about with how well the communication proceeds. If that's there, all the other attributes that others have posted can flourish. Good communication can lead to understanding, which can lead to trust. It all takes time, but worth the wait. Without good communication, there's no foundation on which to build.
I agree that the above posts have covered most of the essential components I would name, trust, humor, mutual respect, love and time. I would add compromise, and a determination for a relationship to work. It is easy to give up when times get hard: both parties need to be sure that they are in it for the long haul. Discussions of the key life decisions - children, location, work, politics, religion - need to be brought up early and resolved quickly. I have two relatives who both ended up divorced as they 'found out' their respective partners had different views on children.
A great number of people talk about finding their "other half," or being "completed" by another person. As #5 points out, each person in a relationship should be a complete person already, and their lives should be enriched by the relationship instead of being entirely based around it. If a person thinks only of himself, or only of his partner, he is not a "complete" person; he is either a tool to be used, or he thinks of the other as a tool to be used. Your life should be complimented by your partner so that both benefit.
Lots of good answers and I cannot disagree with any of them. However, I would like to add my two cents. Great relationships are build on time. You need to spend a lot of time with people and relationships grow and mature in time. This is an important point to make, because in our world everyone is busy. What we need to do is make time.
Trust, sharing, honesty, and good humor have to rank at the top of my list for maintaining a healthy relationship. Mutual interests certainly make life easier, and keeping an even temperament and positive outlook when problems arise help to keep the relationship stable.
Trust and honesty are at top of my list as the two most important ingredients of a good relationship. Listening, good communication, tolerance, patience, respect, camaraderie (friendship), interdependence, and a good sense of humor round out the rest of the pack.
A couple has to find the delicate balance between serving themselves and serving others. There should be a mutual give and take, sharing of responsibilities, and allowing of free expression and individuality. Avoid arguing and getting angry! Just remember this maxim, "In every situation, there are three sides to the story: his side, her side, and the right side!"
Mutual respect is an important component of a healthy relationship. You can usually gauge the amount of respect in a relationship based on the conversation between people. Is there an equal amount of listening and speaking? Is respectful language used that shows the value of the other person in the relationship? Also I believe communication is key. A couple can have trust, respect and love but still fail due to simple lack of communication.
Another dimension of a healthy relationship is that both people in the relationship are able to remain themselves and are able to function autonomously. If we are attempting to make drastic personality changes, which do not work, in order to have a relationship, the relationship is likely to be doomed, and if we cannot function independently, resentment often builds. People need to be themselves and have time apart.
Another critical component of a healthy relationship is a healthy sense of humor! It is important to be able to enjoy each other's company, to laugh with each other often and heartily, and to laugh at each other in kind ways. By that, I mean that I can laugh at my husband's occasional clumsy moves when we are together and alone, but I would never make fun of his missteps to others.
For me, the main attribute is that a healthy relationship is built on trust. In all healthy relationships (whether it be a romantic relationship, a parent-child relationship or what have you) there is trust between the two partners. Each person knows that the other may screw up from time to time but that the other's heart is in the right place and that the other partner will never intentionally do something to harm the relationship.
Certainly this question is open to debate, as there is no formula that would guarantee a relationship to always be in great terms. However, one might want to consider what the experts see in BAD relationships in order for us to filter and avoid bad choices that may ruin relationships. This applies to both casual, personal, professional and family relationships.
The first denominator for a healthy relationship is, undeniably, open communication. If you notice, most seminars in qualitative customer service, dating services, church counseling services, and even therapy, consider that communication on both parts must be open and impartial. This means that the parts involved are able to speak their minds, state their wants and needs, and share their opinions without the fear of being censured, criticized, or judged. Conversely, a bad relationship is always characterized by the lack of speaking and listening, which in turn, leads to confusion and assumptions that should not exist.
The second denominator is mutual support. In any award winning organization the members and sub-groups share a common goal. If you are expecting someone else to move you toward a goal, you are literally fried. It is the mutual want to achieve a common achievement what keeps the synergism alive between the two parties. Imagine a marriage where the husband wants to spend his fun days with his wife, and a wife that wants to spend her fun days with her friends. That is not mutual support. That is not communicating to express the real needs of the couple.
Finally, one last important factor is the ability to make commitments. This has nothing to do with engagements, marriages, nor promise rings, but commitments that show that both sides of the relationship can go above and beyond the expectations to help each other succeed. We do this with our children most of the time- when we take time to help them instead of doing what we want to do, when we shut down our TV to listen to their drama...even when we go into debt in order to provide them with a good education.
Those are just some factors that play a huge influence in the process of developing and sustaining a healthy relationship.