For me, it was everything in her vocals! -When her voice was in excellent form, there was no one from her era that was better....https://youtu.be/-RbGli0juiw
In that video of her in the green, look at the sailors. All those military folks turned into GWORLS! She really had the ability to make anybody cry, scream, faint, stop, jump...Just everything. She was never showboaty with it, which means she was confident in her Skills. She never did too much but the effect of what she could do was HUGE. A small subtle vocal run from Whitney will give you chill but you won't outright notice the run. Shes so subtle with it but powerful as hell. I wish I could sing like that!
Whenever, I'd see that handkerchief or when she would turn her lips just so...you knew, I KNEW some sangin' was ready to pour out. She was not only a MASTER vocalist BUT a great performer as well because she had such presence and dramatic flair. Again subtle but powerful.