With a few more additions, lemmy could serve as a good replacement. We already have a Forum / NewComments sort which is perfect for question / answer type communities. We could add a feature to make default sorts for specific communities, so they would feel less fast, or possibly a sort that brings zero comment posts (IE meaning unanswered), to the top.
The reputation and "accepted answer" features from SO are a lot less important than threaded comments can be, especially since questions often need new answers every year, making the "accepted answer" pointless.