What makes up a piece of content's quality? We consider a number of factors that are used by algorithms and marketers worldwide and provide a subsequent quality score:
The total number of words in the piece of content. You don't want too few words but you also don't want too many. Remember, the goal with content is to be consumed - if you don't write enough or give your readers too much to handle, they may be turned off, even if you have a compelling message.
A basic rule of thumb for SEO is including images. We're not analyzing anything more than the total number of images but this information is still valuable. Do the most popular pieces of content contain three images? Do longer-form pieces get shared more when they have more images? Keep considerations like this in mind when reviewing image count information.
Readability is the ease with which a reader can understand a written text. It is more than legibility or comprehension. A few causes of low readability scores include run-on sentences, sections of content that aren't broken up by headers (H2), and grammar issues like passive voice. Here's a link to the Wikipedia article about Readability that provides much more information about the history and logic of readability scores.