A good biography reads like great fiction: the story moves along at a rate that engages the reader. The style must be journalistic while being graceful. It's tricky business. Being "worthy of interest" has nothing to do with it. A poorly written bio of the most fascinating person goes unread.
I think what makes a good biography is first writing about a person whose life is worthy of interest. If a person has outstanding accomplishments, has successfully overcome adversity, or has even been surrounded by scandal, he or she would be a subject that I would be interested in reading about.
It should be well written. This means it is interesting, covers most of the person's life, what shaped their minds, their experiences from childhood to adulthood, what jobs they've held, what they did with their life.