Often, someone asks a question, gets one or more good answers, and then leaves without ever accepting one. I would suggest that after a certain amount of time has passed that users of sufficiently high reputation should be allowed to vote to "administratively accept" an answer that appears to answer the question correctly and completely (or as completely as possible). This would clear the question from the unanswered list without blocking new answers.
As Mad Scientist noted, accepting is not required to get a question off the unanswered list, but there's something else, I think. It's just disturbing to see a perfect answer, essentially impossible to improve, go unaccepted forever.