Which factors of attraction are most likely to contribute to the success of a long-term relationship?

There are a number but in my opinion the two main components must be respect and trust. Respecting the person that you love be it with what they do and their time will take you to places. Giving them the life they have before you came along and respecting each other's boundaries is very important. Which moves us to the next. Respect. When relationships tend to be long enough with out butterflies and sparks. The only thing that is left should always be respect. No matter what happens, give the other person the ample amount of respect that you want to be given back to you.