Nadi dosha
Nadi is the biggest koota, at 8 points. Every nakshatra belongs to one of three Nadis: Aadi, Madhya or Antya. If both of you share the same one, that is Nadi dosha, and the koota scores 0 of 8. There are only three Nadis, so for two people picked at random, about one pair in three shares one. Families worry because tradition ties Nadi to the health of the couple and their children, and because losing 8 points lowers the total.
The calm view: weigh Nadi with the rest of both kundalis, and don't let it decide alone.