I've noticed for a while that sometimes someone will answer a new person's question, but that person will forget to mark it as answered, even if they are reminded to. I have a feeling this happens because people just use the site once and once they get the answer they want they never come back. There should be an option for people with high reputation (I.e 3000+) to mark the answer on a question that has been abandoned for ~7 days. Perhaps maybe if someone who posts the answer could request it to be looked at, and see if it was just a neglected answer or not.
A question is considered answered if it either has an accepted answer, or has an answer with positive score. So, the feature you propose already exists: every user with 15 or more reputation points can mark a question as answered by upvoting an answer to it.
The sole meaning of the "accepted answer" checkmark is to indicate what answer the question's author preferred. Allowing someone else to place would destroy the meaning of the checkmark; it would be more logical simply to remove the checkmark feature. The community's opinion on what answer is best is already expressed by their votes.
- Would it be possible to have a “community accepted” feature?
- Is it possible for someone with high reputation to mark as answered?
- Allow users to accept answers on questions they didn't ask
and so on. This feature keeps being proposed and keeps being declined; by now it's in the same category as "require comments with downvotes".