Portfolio formal elements

Definition 

Portfolio:an edited collection of a photographer's best work intended to showcase the style or method used in their photography. A portfolio is used by artists to showcase their versatility by showing a range of samples of current work.

In my opinion, what makes a successful portfolio is the photographs fitting the criteria for the element. For example, the element might be 'tone', so a photograph for 'tone' would portray the contents of the criteria from the definition of tone, such as showing a range of tones in one picture. Layout of the photographs will also make the portfolio successful, for example, the portfolio will be better if the images are organised rather than scattered about.