It can be any personality trait that you find particularly irresistible.
For me, it's the "face scrunch". Every time she does it while singing, I swoon. I mean, I become putty in her hands.
She did it from the start of her singing career, but even in one of her more recent videos (Je veux bien), she does one that kills me.
It's right about at 2:29..
Do you think it is something that might have been taught to her by MF, or is this natural?