When dealing with social bookmarking sites like digg you have to take into account the massive amount of content that is going up every day. Look at the popular posts. Check out their titles. From my experience, put something up that's controversial, it will stir reactions on both sides of the controversy. Also I've noticed that documents which also get a good amount of attention specifically on sites like digg are things like Top 10 lists, How to, etc. If that's in the title, depending on what it's about, it can boost the amount of viewers significantly because people are always looking for quick reads on these sites, so lists are quite popular. If you really want to get deep into it, take a look at things like "Propaganda" by Edward Bernays. The public relations industry is basically all just a study and the artform of propaganda, and how to use it to mold the ideas and get the response you want out of a group of people.
Top 10 lists of pop culture subjects like movies, babes, cars, etc. Shocking/disturbing headlines about breaking news, videos Blog posts that are teh fail Astronomy picture of the day, the latest Funny or Die video or one from College Humor
in social bookmarking sites such as digg, you only got a minute to capture the attention of the people over the network. So it should be catchy, ideas are things that are controversial, funny stuffs, inspiring or even educational. But basically it depends on how you write up the title.