I have noticed a couple of times that when I answer a question it is fairly obvious the original poster (OP) has upvoted my answer but they then don't accept the answer - looking at when they were last seen and when you get upvoted is a fairly good clue. It seems most often to occur with new/low usage members hence I'd suggest that sometimes they don't know that they are supposed to press accept if an answer is satisfactory. This leaves some questions as "unanswered" when really they are answered which isn't good for the site.
So as a potential solution would it be possible to have Stack Exchange recognise when a user upvotes an answer to the question they posted and then remind them with a speech bubble or popup window to also accept if the answer is good? (popups could be just like the one that comes up when you try to change a vote a while after your original vote and tells you the post must be edited before you can change vote).