Dear Samantha, 
 Many thanks for the email, and I hope that I can address your questions satisfactorily. The four most useful observations (I'd say that rather than "corrections"--these might be active voice or effective sentence structure [i.e., the rhetorical difference between complex and compound sentences]) for non-fiction would be 
1) integration. Especially in longer pieces, authors can sometimes overlook opportunities to have different parts of their text resonate with each other, thus creating an organic whole; there's not much difference between a well-plotted novel and a well-written dissertation in this sense. Conversely, writers in longer pieces can sometimes re-introduce material already covered, thus giving a sense that the writer has lost control over the text's direction. 
2) Voice. It's tempting to write in "academic-ese" but effective writing usually stems from an author's idiosyncratic style: this is what readers respond to and what they remember.  
3)  Acknowledgement of complexity. The best, most engaging writing is that which knows that what's coming next is going to be complicated and it's this complexity which is what the reader is looking for and anticipating. I look for places in manuscripts to raise questions or to challenge assumptions. Good readers, especially in academia, will do this, and this can affect not only grades, but publication decisions. As a writing teacher, the most common mistake I find in student writing is when students try to make a complicated issue as simple as possible. While its clarity can be a virtue, its overlooking of the complexity of the issue can be underwhelming. 
4) Concision: good writers know that good word choices and more lively writing is far more effective than longer, but less informative writing. So I look for opportunities to cut out words, phrases, and even sentences that don't add anything to what the writing sets out to do. In a way, this is connected to voice and eliminating the kind of writing that doesn't actually do anything. It's a bit like those essays wherein the writer will start with "Throughout history, people have always wondered about history and there have been many positives and negatives about the roles history plays in society." While it's a perfectly grammatical sentence, it's so vague and broad that the reader has little sense of what's coming next. 

So, this a brief summary the most important corrections/observations that I make. In all of these, I look to make the piece more engaging, for the ideal reader is looking to be engaged, to be entertained (in a way), and to be exposed to an idea that may not have been so obvious before: the "that's interesting; I'd like to know more" reaction that means the reader is actively interested in the piece. 

Best Wishes.