Activity
#Activity IfIf a user is not active, he can not fulfill his duties as a moderator.
Helpful Flags
#Helpful Flags AA user should be able to identify when a post is bad. Having lots of disputed flags would be a good indicator that he is a bad judge of quality.
High Votes
A moderator should not only smash down the ban hammer whenever someone makes a mistake, but also encourage good users by voting on their posts.
Many Reviews
Having many reviews shows that you are willing to help monitor and improve the site. If you do not review posts normally, how could anyone expect you to do that and much more as a moderator?
Good Standing
Any prospective moderator should have a good standing with the community. If a moderator never answers or asks any questions, never comments on posts, and never visits the site's chat rooms, there will be a void between the users and the moderators. It will be much harder for them to keep the site's quality where it should be, without discouraging users away from the site.