We have a reasonably clear sense of what "good" is, but it's not perfect. Suffering is bad, pleasure is good, more people living enjoyable lives is good, yes, but tradeoffs are hard. How much worse is it to go blind than to lose your leg?  How do we compare the death of someone at eighty to the death of someone at twelve? If you wanted to build some automated system that would go from data about the world to a number representing how well it's doing, where you would prefer any world that scored higher to any world scoring lower, that would be very difficult.
Say, however, that you've built a metric that you think matches your values well and you put some powerful optimizer to work maximizing that metric. This optimizer might do many things you think are great, but it might be that the easiest ways to maximize the metric are the ones that pull it apart from your values. Perhaps after it's in place it turns out your metric included many things that only strongly correlated with what you cared about, where the correlation breaks down under maximization.
What confuses me is that the people who warn about this scenario with respect to AI are often the same people in favor of futarchy. They both involve trying to define your values and then setting an indifferent optimizer to work on them. If you think AI would be very dangerous but futarchy would be very good, why?
I also posted this on my blog.
 This is a question people working in public health try to answer with Disability Weights for DALYs.