It seems that many questions have answers, but those answers are not accepted. This is problematic for two reasons: First, when an answer has been provided, when searching for unanswered questions to contribute to, many already have acceptable answers. I, of course, up-vote those answers, but it's still annoying.
Second, it gives the impression that the answer provided is not authoritative or correct. In many cases the question is politically motivated—especially those about international security—and the person who asked the question may not like the answer they got. In other cases, the person asking the question may only be a casual visitor and either not know to mark the question answered, or not return to see the question is answered.
I am unclear on exactly who, other than the person who asks the question, might be able to accept an answer. I have been on the board for a while, but only answer intermittently, so my rep is not particularly high, and consequently there may be tools available to mods to resolve this issue. Also, this may be something that we may want to encourage in other ways.