Mark places where the topic sentence seems unrelated to the rest of the paragraph, or where the argument seems to drift away from the thesis. Introductions and Conclusions are the hardest parts of a paper to write. Help your author brainstorm if she or he is having difficulty framing the argument. Be polite, gentle but firm in your criticisms. Instead, point out how much you like the thesis, how strong it is, and then suggest that the rest of the introduction detracts from it in comparison.
Authors are usually grateful for reviewers who notice grammatical, typing or spelling errors.
Point them out and mark them on the paper. Make allowances for style by all means, but don't be shy to point out usage errors. Take full advantage of your reader's editorial comments and suggestions, and of discussion within your group, but make sure that you do not inadvertently plagiarize from your group members or from their papers. Finally, if you are in doubt about your paper's originality, ask me.