What is the main idea the writer is trying to get across?
Is the writer trying to be humorous, serious, straightforward, etc? Does this emotion/tone work for the piece?
Who does the piece seem to be written for? Who is the audience?
Does the writer give enough details and examples to illustrate what he/she is trying to get across? Where are there good details? Where could more be given? Are there any places where the details slowed your reading or bogged you down?
Does the piece follow a logical order? Are there any places where you got lost? Are there details that should be given earlier or later in the piece?
What type of piece does the writing seem to be - an article, an essay, a column, a book or book chapter?
Overall, what did you like or not like about the piece?
Keep in mind that nonfiction writers write for a variety of publications including magazines for articles, online columns, newspaper editorials, etc. Try to put yourself into the mind of the audience to make the best critiques.