How are featured questions sorted on Stack Overflow? And can I sort them using different criteria, such as by posting date?
Since September 2011, questions on the featured tab are no longer only sorted by the expiry date, but the bounty amount has some effect too. From the blog, Bounty Reasons and Post Notices:
There are actually two featured lists: one when viewing Top Questions, and another when viewing All Questions. Bounties with a (very?) negative vote count, are not shown on the first (the list will even shrink if there are few other active bounties), but will still show on the latter.