A new study published in Personality and Social Psychology Bulletin this month reinforces a message that International Hobo have reported in their previous player studies: players aren't necessarily looking for violence in videogames, and different players are looking for different things. The study in question singles out challenge and autonomy (i.e. player choice) as key themes.
You can read a potted account of the research on Psych Central here, the abstract is here, and the full paper in PDF form is here (for subscribers of the bulletin only, alas, or $20 on a "pay per read"). Here's an extract from the summary of the research:
These elements, said coauthor Richard Ryan, a motivational psychologist at the university, represent “the core reasons that people find games so entertaining and compelling. Conflict and war are a common and powerful context for providing these experiences, but it is the need satisfaction in the gameplay that matters more than the violent content itself.”
This study is the most comprehensive investigation into this subject, and should be of great interest to both academics and commercial game developers.