Basically OP, the human body is 3 dimensional, and fabric is 2 dimensional. It's quite a challenge to transform a bunch of flat pieces of cloth into a garment that fits and flatters the human form.
You'd think that fashion labels would have nailed it by now, but as fashion is always in flux, the cut of the garments in a collection will always be changing too. So each season there will be some styles that just "work".
In practical terms, I'd say that it's much easier to recognise a good cut in a woven garment. So a jumper, a tshirt, a knitted dress or a jersey skirt may look slouchy and cool, or soft and cosy or whatever, but is unlikely to inspire that "great cut" sort of response.
Whereas a perfectly fitting shirt, smart jacket or flattering tailored dress will give that smart, well-cut look. The shoulders of a garment are an area that can make or break the look - armholes are notoriously difficult to pattern-make and sew, but when it works, it'll make the whole outfit.
You know it when you see it. Basically, if the length/shape/fabric makes you look shit - avoid!