…nothing actually. I love net promoter score, but I do think there could be an opportunity to improve NPS.  I read someplace once that a customer with a great experience will on average will tell 4 people about it, but a person with a bad experience will tell 10 people about it.
 I don’t have any research or supporting data on this other than one data point, me.  For me, I definitely tell more people about a bad experience than I do about a great experience.  When I had some significant difficulties with our home security provider (they could only do their software updates at 3am in the morning which caused the system to beep repeatedly), trust me, everyone around me heard about. I am also a person who makes a conscious to let people know about great experiences as well, but even for me the ratio of negative referrals is definitely greater.

Long story short, NPS should be updated to reflect how influential really negative reviews affect an organization.