In the last few years, we have designed software applications for multitouch wall displays which provide information and/or services in different contexts, such as cultural heritage, tourism, and public events. While many of the systems proposed in the literature concentrated primarily on entertainment purposes, we are moving towards more functional applications and we are planning a set of empirical studies to evaluate their impact once used in the field. In this position paper we describe some challenges that emerged during our work and propose some initial variables which can inform the definition of an evaluation framework specific for public display applications