Miniskirt Monday pic in the morning.
I would say, the major factors - because there are so many, and each relationship has varying degrees of depth to them - would be the following:
TrustTrusting each other to do the right thing for the relationship, the family, the longevity of the relationship, and the person. Trust is one of the hardest things to earn ... and one of the easiest things to lose in any relationship.
FriendshipBeing good friends is extremely important in the development of a relationship and adds to the foundation of trust. That's not to say you have to be identical, with identical interests, but you have to "like" your partner, as well as "love." There is a big difference. I'm lucky because I married my best friend, and that friendship has grown even stronger.
HonestyWithout honesty, there can be no trust. It is the cornerstone of a lasting relationship. And while everyone has secrets, there's a big difference in hiding, say, an affair and hiding the receipts from your last dress-shopping excursion. The important part is having the ability to communicate and share the differences in ideas, desires and dreams.
CompassionYou need to have a heart, and understanding and a willingness to open yourself up when most vulnerable.
Kindness/RespectReally, I think this goes without saying. It's the mortar that holds the foundation in place. I could never be with someone that did not respect who I am, and be supportive of my ambitions, dreams and goals.
AdventureFor me this is important. On the surface, I may look like a pretty average hockey mom/housewife to people, but I relish adventure and it plays a huge role in defining who I am. Without my partner sharing my sense of adventure, the relationship could not evolve constantly. Not to mention how dull life would be without it.
Whew ... my brain hurts now.