During the process in groups of identifying what practices makes a good graphic design I was able to first see that we all had different perceptions of what is correct and what is not.
Although there are rules that are applicable, as after all, visual communication is a form of language and if we all do not speak the same it would be very confusing.
The first important thing to highlight is that every design and every decision in every design should have a purpose. Nothing should be arbitrary, it would be like saying random words while one speaks because it feels impossible to find the right ones.
Basing this identification on the idea that "everything is ok, as long as it is justified" everything else falls in place.
The shapes (typeface or other geometries) one wants to use should be according to the message and tone of voice. If a designer convey a clear message or one very difficult to read.
The chosen format (digital, printed on regular paper, on thick paper, glossy, etc) needs to go also with the message and tone of voice. For instance, if one thinks about something glamorous on paper it would be glossy. Or maybe not, but always it needs reasoning behind those decisions.
The awareness of figure and ground also helps to show the message that is wanted to be conveyed. Does it need prominence or all the opposite? Should it look relaxed or tense? Grids also help to bring order to chaos, to structure this particular issue establishing align the information through repetition and consistency.
And last but not least, the colour. A designer might want to convey coldness with red, but why? There must be a good reason behind it, and also a way that allows the audience to understand what is going on. Or maybe not. Maybe the designer's point is to make something impossible to understand... but again, as long as it is justified, the awareness of colour theory must be reflected even if one decides to follow the general rules or not.