EXCESSIVE EMPHASIS! is what happens when a writer uses EVERY MEANS POSSIBLE to EMPHASISE the text, hoping this will get across that what is being said is REALLY, REALLY IMPORTANT!!! It gets very hard to read, and is particularly annoying when what's being said actually isn't important.
Often involves italics, bolding, CAPSLOCK OF RAGE (or Fear, or Joy, etc.), and long strings of exclamation marks!!!11!!!one!!!1