We explore how platforms influence user-generated content by comparing reviews made on the retail platform Amazon, with those on the (non-retail) community platform Goodreads. We find the retail setting gives rise to shorter, more declamatory, and persuasive reviews, while the non-retail community generates longer, more reflective, tentative reviews with more diverse punctuation. These differences are pronounced enough to enable automatic inference of the platform from which reviews were taken with over 90% F1. Significant differences in star-ratings appear to parallel differences in review content. Both platforms allow users to give feedback on reviews. Experiments show that a subtle difference in the review feedback features influences review-promotion behavior, which may in part explain the differences in review content. Our results show that the context and design of a platform has a strong but subtle effect on how users write and engage with content.