This experimental study investigated pre-service and experienced teachers’ formative feedback responding to upper-secondary English as a Second Language (ESL) argumentative essays. It examined differences in feedback quality and linguistic features regarding teaching experience and text quality (high/low). We developed holistic criteria of effective formative feedback based on empirical findings in order to rate comments by 26 experienced and 41 pre-service teachers. Natural language processing tools were then applied to evaluate linguistic features of these comments. Results indicate that teachers provided more high-quality feedback to stronger essays than to weaker texts. No significant difference was found between pre-service and experienced teachers in terms of feedback quality. Further, comment length and absence of negative adjectives seem to predict feedback quality. Implications for research and practice are discussed.