What do you think is important in keeping a BDSM relationship healthy?
Love. Trust. Honesty. Communication. A healthy sex life (at least for her). Friendship. Companionship. Quality time. Intimacy. Desire from both parties. Effort. Care. Attention to detail. Planning. Shared interests.
How does it differ from a vanilla relationship?
I don't think it differs at all really, assuming we are talking about a healthy vanilla relationship.
The only difference is that with D/s, the expectations are clearly defined.