From its origins in the mid 90s, the application of the concept of virality to commercial communication has represented an opportunity for brands to cross the traditional barriers of the audience concerning advertising and turn it into active communicator of brand messages. Viral marketing is based, since then, on two basic principles: offer free and engaging content that mask its commercial purpose to the individual and using a peer-to-peer dissemination system. The transformation of the passive spectator into an active user who broadcasts advertising messages promoted by sponsors, responds to needs and motivations of individuals and content features which have been described by previous research in this field, mainly through quantitative methods based on user perceptions. This paper focusses on those elements detected in its previous research as promoters of the sharing action in the 25 most shared viral video ads between 2006 and 2013 using content analysis. The results obtained show the most common features in these videos and the prominent presence of surprise and joy as dominant emotions in the most successful viral videos.