There can be many different answers to this question because different people will have different values that are emphasized in long term relationships. I tend to think that emotional attraction is one of the most important for sustaining a long term relationship. If statistics are right that nearly one out of every two marriages end in divorce and so many couple break up, it seems that being able to find some level of emotional connection or attraction with another person helps to sustain a long- term relationship. Physical beauty fades, so it cannot be relied upon for a long- term basis. Material wealth can either run out or be unsatisfying in the long term. Intellectual attraction might not satisfy the basis for a long term commitment to another, as one can always "intellectualize" their way out of attachment.
I tend to think that the elements of emotional attachment help to establish the basis for a long term relationship. Emotional attachment is essential for a long term relationship because one falls in love with the core being of an individual. One learns to love the others' hopes, fears, failures. There can be no better statement for a long term relationship than one person saying to another, "I know everything about you and I still love you." This is an emotional form of commitment, an attraction that is everlasting. As the recent debate about gay marriage has taken so much importance, hearing gay couples who wish to be legally married talk about being "open" with one another and being emotionally connected to one another helps to form one basis for a long term relationship.