What should I be looking for if I must write notes on the Elements of Style, like language, organization, diction, or any other stylistic devices?
I'll give you a basic idea of what each of these elements of style include - and hopefully you can take it from there. I'm going to take them out of order because I think it might make more sense this way.
Diction is word choice. Here you want to look for any vocabulary that holds a very specific meaning or connotation. Consider the difference between describing a child as "chunky" or "husky" compared to "fat" or "obese." Although each of these words has the same basic meaning, there is a reason why you might choose one over the rest depending on the message you wanted to relay. (Do you want to paint the fat kid in a positive or negative light?)
Language (to me) could mean a few things as opposed to just diction. With language I would look for figures of speech or figurative language. These are similies, metaphors, personification. When an author is being figurative this means he or she is using non-literal descriptions for things to emphasize something. "I'm so hungry I could eat a horse," is a common figure of speech. I couldn't really eat an entire horse - duh - but it a huge animal that is compared to my huge hunger.
Organization is a broader look at either a section, chapter, or the novel as a whole. I suppose in this novel you might want to ask, Is it organized logically? Chronologically? Does it follow a basic story line (exposition, rising action, climax, resolution)? Are there flashbacks? How many plot lines are going at once? Where do they intercept?
One other major element of style that is left out of your list is tone or attitude. I think it is pretty central to how you look at the rest of the above. Ask, What is the author trying to say (what message is being sent) and what tone is used to speak that message? Why and how? This is a central question to style analysis that you can use to incorporate all these other elements.