My relationship with my spouse has always been so open, loving, respectful, that after 17 years of relationship I've had little reason to think of deal-breakers. My relationship with my boyfriend is also natural and positive. My husband's relationship with his girlfriend is also quite positive.
HOWEVER, my husband's girlfriend's other boyfriend (yikes) has demonstrated to me things that I could never put up wtih in a relationship. They are ultimately the following:
Controlling behavior in any form, including emotional control
Holding another person responsible for working on one's own feelings
These are more in the line of "behaviors" than values, I suppose.
"Healthy" to me ultimately means a good balance between connection and individuality.