There's a lot of things that can be appealing about a photo. When it comes to portraits it could be a mood or emotion. It could be some cool styling, possibly using a location and nice lighting to create a scene. There's really no one thing that works as every portrait is different, and every shot is trying to show something different.
For me, when it comes to portraits, my personal preference and the thing I like to see the most....is a beautiful natural smile, and Dani manages to demonstrate that perfectly here.
This was taken on my Nikon D810 with Tamron 70-200 f2.8 VC lens.