One of the main reasons that being fat (e.g. having high BMI) is bad for you is that it screws with your metabolism.
A new paper directly measures the metabolic state of people with various BMIs and found that there are some obese people who are metabolically healthy, while there are people with "healthy" BMIs who have screwed up metabolisms. This flows, in part, from genetic differences between people of a given BMI.
The paper implies that we should measure the metabolic state of obesity patients and treat them differently based upon the extent to which they have healthy metabolisms.