Most posters for presentations are made by amateurs. They are made by researchers or students or other professionals who have never learned how many picas to an inch.* That’s perfectly understandable, and I don’t expect every poster presenter to embark on a detailed study of typography before drawing up a poster. But recognizing that there is a body of knowledge, a discipline of serious craft around typesetting and layout created by disciplined practitioners must only help.
Here is a list of things that make professional typesetters cringe, over at FontFeed. This list includes capitals for emphasis, incorrect dashes, dumb quotes (smart quotes shown), and more.
The one guideline that is contentious for poster layout is whether or not to justify the text. On a large poster with multiple columns, there may be advantages to the crispness of justified text. The trick becomes finding a good balance in column width and font size. More on this later.