1. Questions sorting by time:
Questions > Most votes > ...of week, of three months, of year, all time;
Questions > Most answers > ...of week, of three months, of year, all time;
Questions > Most views > ...of week, of three months, of year, all time.
1.2 Sort questions by: Most votes, most views, most answers, unanswered, hotness at /search, /user, /tag pages.
2. Option to completely disable downvoting or require x amaunt of points to downvote.
4. Optional moderation mode (admin approval before anonymous post appears)
5. Stop users from the same IP address creating multiple accounts, ignore voting from same IP adress to post created by the same IP adress as voter voting.
Sorry for English language skills and thanks Gideon !
I have some more, not in list, not prioritized, maybe someone think they are important and ad to theyr list:
Option to set category where questions with no category would go.
See who rated post by hovering on rating.
(1.3 search question in particular category).
Update/renew button to renew question for example onse a day, so it can be seen on top of the questions as "updated".
Option to hide or coloring text to grey answers below -3 rating (By default or just by user)
More interaction for questions for example random questions widget, unanswered questions widget, all user's questions, all users answers at user page.
More advanced private messaging system.