Is to be able to give and receive total trust.
Without trust no relationship, whether vanilla or BDSM will work. It is even more important when you give control of your body over to someone else.
Besides trusting that we will not be damaged too much phyically, we need to be able to trust our doms to take care of our needs as well as their own.
Complete honesty is essential to make a D/s relationship work, and trust can only grow from a sound base built on that honesty.