The Global Game Jam 2013 diversifiers have been published. This year, there are things like “Stay Inside The Box”, “I Know What You Did Last Summer”, and “10 little pigs”. See what these mean and the full list from the GGJ site.
Unlike the theme, diversifiers are completely voluntary, and all teams may choose and pick any of them, or many of them, as they like. There are no points given for using the diversifiers; they simply aim to diversify the games and give extra motivation to more experienced teams. So if you’re a team of first-time jammers with still little experience on game development, it might be better to focus on the primary constraint, the theme.