Online communities are increasingly important in forming and maintaining relationships but have also faced criticism for toxic or depression-inducing content. Motivated by research showing that grateful reflection can lead to enhanced well-being, this paper explores how we might design online communities centered on gratitude through a qualitative study with 15 participants. To elicit insights on how people view expressing gratitude in online communities, we built a simple gratitude-centered online community, Gratitude, designed to be similar in nature to many online communities, where users create short posts (notes of gratitude in response to prompts) that other members can browse, react to, and comment on. Participants were first interviewed and then used the Gratitude platform for three weeks, during which they filled out surveys after each prompt. We found that while participants had concerns or questions about the value of expressing gratitude publicly and the risks associated with doing so, they also described experiencing many benefits, such as providing one with a platform for sharing; inspiring reflection and positivity; fostering connection and empathy; and contributing to a cycle of gratitude and vulnerability. Participants raised several areas for design related to better support for interaction and connection, privacy and authenticity, and motivation and engagement. We conclude by discussing implications for future research on the design of prosocial online communities centering cycles of positivity and authentic individual reflection in communal interactions.