This is a little bit of an odd question... It is not a specific thing or feeling that makes a relationship deep, it is a compatible set of specifics shared between two people that make the relationship deep for them.
Yes, BDSM is a part of my life and submission an aspect of my personality - the trust required to practice it with my partner is a draw for me and deepens my connection with that partner. However, my BDSM relationship is no deeper or more meaningful than the partnership of my neighbors, who bond over their mutual spiritual beliefs.
Vanilla may seem shallow and unfufilling when you're the lone BDSM'er in the couple, but BDSM can seem as equally shallow to a person whose soul is touched by another writer, or a musician, or someone who simply shares the same family values.
No set of values or traits are deeper than another all on their own - the depth of a relationship is measured by how far into our hearts we can take our partners with us, and how far we can travel into theirs.