Stack Overflow changed the way they sort answers:
We no longer pin the accepted answer (with the green checkmark) to the top of the list of answers. By default, we now sort strictly by votes (descending order by highest score), and the accepted answer's order in the list is based on its score.
and it is asked on Meta Stack Exchange if we would like to have the accepted answer unpinned on our site:
We can change the way the engine sorts answers in site settings. We would like to hear from you all if it is something you want to see on your site. ... We are going to collect feedback before the end of September 19th.
I am posting this here so that we can have a discussion of what is best for the Mathematics Stack Exchange site.
Update:
Based on the voting and the report on Meta Stack Exchange, the site setting has been adjusted to not pinning accepted answers on Math.SE anymore:
There's no need to do this on the sites that are on the list already; we have adjusted the site setting for them.
