The Searchlight Digital blog has a detailed analysis of three weeks of Sphinn frontpage success. The analysis takes a look at the top contributor and the categories in which the stories became popular.
From looking at that data, the conclusion (besides being a power user) is obvious:
If you want to kick ass on Sphinn, there are specific topics you should be writing on, namely SEO, social media and Google.
Well, that may be true, but the Sphinn discussion says that the other sections (e.g. Usability) have less submissions overall so there's a higher percent change that the usability stories will frontpage versus a SEO submission (among submissions in the same category). At the same time, those topics don't have submissions in them because there's a lot less interest compared to more general topics.
Forum discussion continues at Sphinn.