When giving blood, i was told i have rare antibodies that make my blood in high demand. i'm o+, which i understand to be common but not rare. what do they mean by rare antibodies?

Special blood: In addition to the "basics" (A/B/O and rH factor) we all learned in school, there are about two dozen "lesser" blood antigens (/"groups"), and the presence or absence of some of these can be very important, usually to a patient that has had transfusions in the past. Examples of these are "Kell" and "Duffy," see table: http://en.wikipedia.org/wiki/Human_blood_group_systems