Commitment. If you don't care enough to put in the effort, no point.
Trust. If there is no trust, there is no relationship.
Faithfulness. Infidelity is a lack of commitment and trust.
Communication. If you can't speak to each other comfortably and resolve issues quickly and appropriately, long term is unlikely.
Support. Do you have an outside support system or a life outside of the relationship, so that you live as one person instead of half?
Compatibility. If you aren't compatible in several areas of the relationship, it most likely won't work out.
Common interests. Is the whole relationship gifts, conversations about nothing in particular and intimacy, or do you have hobbies and activities that you share together? Something just for you as partners? This is the foundation of a stable relationship.
All I can think of for now.