I spend a good amount of time studying painted portraits (love browsing A Stroke of Genius website ), where the primary intention of the image is to be a legacy portrait that will hang on the wall for generations.
The most basic similarity is, of course, that they are in hard copy--physical form. Another is physical size--almost never less than 16x20 and frequently 30x40 or larger. The size is often a function of achieving a reproduction of head size close to life, so a 16x20 is likely a head-and-shoulders portrait while a 30x40 may be half- or 3/4 length.
There is something visually impressive about scale in seeing a good portrait as it nears life size, and that is, I think, one of the factors that make the portrait more likely to be retained.