Proportion, symmetry, scale, layout, etc. I got the tallest, widest wall unit, a huge ottoman and a room-sized area rug. Art is also either overscale or grouped. The rooms in your photos will have matching scale and good proportions. And while you might see a delicate bench topped by a massive painting for dramatic effect, you'll never see a big sofa topped by a few wimpy plates or prints.
Interior designers know the exact science of scale and proportion.
A good one can tell you, over the phone, what size of bed and dresser is going to work in your bedroom. It's advice worth following, but you can get by with a careful eye-balling as you decorate. If company is coming, chances are they'd be more attracted to that formal living room if it has soft music or nature sounds playing, a scented candle burning, and enough heat or air conditioning. A well-done room also gets a boost from the flicker and crackle of flames in the fireplace, as well as the silent swishes of tropical fish in an aquarium. And as for the fifth sense, a bowl of candy never hurts.