I think she had a beautiful voice (however I am not an especial fan, much less a connoisseur, of the jazz voice, so I will abstain from the trite comparisons).

But I think what carried that voice and her music to a higher level than some people here want to give her credit for are her lyrics. I think she was a gifted lyricist who found words to express experiences in her life - experiences that many people heard and related to because, quite aside from her exceptional drug troubles, what she described with an uncanny insight were very common experiences. Sometimes she used words in a crude but amazingly "that hits the nail on the head" way, sometimes in ways that retreated to very personal but still evocative symbolism. The long and the short of it was, the subject matter might not appeal, the mode of expression might not appeal, but it was blatantly clear to anybody who listened to her that she sang true words. And for me that makes a lot of difference and puts her in a league beyond "middle of the road"/"mainstream pop", whatever other dismissive label you want to give her.