Poor furniture placement.  Avoid pressing all of your furniture up against the walls as if there was a dance floor cleared in the middle, but also make sure you can pass through the room unobstructed by pieces of furniture.

A room that is off balance.  A room is off balance when all the large heavy pieces are on one side of the room and all the smaller, more delicate pieces on the other side.

Furniture of different heights. When seating is of different heights and/or artwork is hung at different levels, the effect creates a 'roller coaster' that causes the eye to travel uncomfortably up and down around the room.