It's all about scale
Designers/decorators are like acrobats, always juggling the elements and principles of design to create inviting and interesting spaces. These tools of the trade come easily when you spend your time immersed in design, but for the average person it takes a little more effort to pull together a truly well designed space. One of the the most noticeable problems I see in many spaces is a lack of understanding/ application/ use of proportion and scale. It's a biggie in my book. Playing with scale can be used to great effect, but when done wrong it can creates visually uncomfortable spaces. Proportion and scale go hand in hand since both relate to size and shape. Proportion is about the ratio between the size of one part to another, and scale is how the size of one objects relates to another, to the space as a whole and to the people who will inhabit the space . My best example of disregarding scale is choosing a large overstuffed love seat, chair and sofa for a small space.