I was reading
Jakob Nielsen's article about getting more people to participate in social media, and one of the points he makes is that you have to make it as easy as possible for people to get involved.
I think, especially if you have a new site with not many existing users, it's important to let people contribute without having to go to the trouble of setting up an account. If there are already lots of people using the site, they might be more willing to make the effort to set up an account, but while your site is still new it's unlikely that they will bother, so you have to make it as easy as possible for them to contribute.
For this reason, I think it would be great to have a section in the control panel that allows admins to choose the level of privelege that anonymous users can have, such as:
0) None.
1) Vote on stories only.
2) Vote and comment on stories only.
3) Vote, comment and submit stories.