I am an occasional user of Stack Overflow and just started spending more time on Super User. Completely based on anecdotal evidence but it seems fewer questions with valid answers here get marked as answered as opposed to Stack Overflow.
For example here is an older question with a good response: Stop UAC/Secure Desktop from dimming the screen that was not marked as answered.
Again I haven't analyzed all of the questions and answers between the sites. I guess my question is: Are there social forces that make it more likely questions get marked as answered from one site to the other.