Auto manufacturers usually mount a protective rubber strip on car doors. This helps guard against dents and dings from similarly-sized vehicles. However, if the car you park beside is considerably taller or shorter than yours, a carelessly flung door could miss the protective strip and instead dent the door panel.
Additionally, note the length of the doors on nearby vehicles. For instance, the doors of two-door cars are usually many inches longer than those of four-door vehicles. When parking next to a car with unusually long doors, be sure to leave additional room for them to open.