A lot of it is going to have to do with somewhat flat, soft lighting (very large softboxes or reflectors or whatever, and not-far-from-1:1 ratios of fill and key lights) + the fact that the model is very fair skinned and has a white dress in a light colored room and furniture, etc. also reduces contrast and makes everything look softer.
Apart from that, it looks like soft focus. There are some lenses which will do that for you, or a really easy and effective way to get soft focus in photoshop is:
1) duplicate the photo in a layer
2) Gaussian blur that layer 10% or so.
3) Put that layer on top and make it about 20% transparency (which is about right to make it subtle/not obnoxiously obvious you did it, adjust to taste)
4) using a soft brush, partially or completely erase any portions of that blurred layer where you want good sharpness, like eyes or detailed hair spots, or textured fabrics, etc. that are in focus