I'd add screenshot collages to the list of things to avoid. Instead of showing just one, informative screenshot at a time, some developers stick 4 screenshots into one, making all of them too small to be legible or comprehensible.
agreed. another thing is to put the artworks of the app as screenshot. Games, especially one of those trading card games are often guilty of this, putting the wallpaper kind of pictures as the screenshot, while offering no information regarding the gameplay at all.