In short, a good BDSM relationship (even if the relationship is only in the bedroom/ dungeon) isn't about bondage, discipline, dominance, submission, sadism, or masochism; those are just the methods. It's about openness and communication; it's about comfort exploring one's passions and comfort with the person you choose to explore those passions with; and mostly it's about trust. So pretty much, it's no different from any other good relationship.