As the SE network has expanded and matured, many sites ask questions on topical and constantly evolving issues. Sometimes, new information comes up and people post answers to old questions. However, due to the abundance of incumbent answers with many up-votes, it will be difficult for this new answer to gain prominence and rise to the top.
EDIT: In light of @cwallenpoole's answer, I modified the proposal below from changing the universal voting scheme to just adding a fourth sort-by option.
My proposal: Add a fourth sort-by option (in addition to active, oldest, and votes, for decaying votes. Just as commitment decays over time in Area 51, up-votes for answers would decay over time under this sort. That way, if a new answers comes in after a year and receives 2 up-votes, that could count the same as, say, 4 up-votes for an answer posted in the first few days after the original question was posted. If popular, this could perhaps become the default sort for older questions.
Note: I am not proposing that the reputation gained from an answer decays. Just that the answer shown as the top (or second-to-top if any answer has a check-mark) be modified, as well as perhaps showing the decay-modified vote count somewhere. Perhaps this last bit could be a privilege, similar to the privilege to see up-votes and down-votes separately.
Showing most recent answers first in old questions would also go part of the way towards resolving this issue, but it seems not to have been adopted.