Role Model - Always
Moderators set the tone and expectation for a community. A moderator who is rude, incomprehensible, unruly or unintelligent can lower expectations and confuse new users. This applies to all circumstances, chat, meta, main, comments and even other network sites. The moderator should be someone who is looked up to. Look to community managers for example. Every time one posts on meta, or makes a statement, it clears up confusion and calms irritate users. There thoughts are usually both correct and final; and they communicate them in a way that everyone understands without fail. Wow, I wish I could be like that.... every time I read there work, I think about how I could communicate better. Those types are who our moderators should be.
Dedicated - now and in the future
Maintaining a site takes work, and these individuals need to be prepared to do it. That is pretty obvious. But, as we have learnt the hard way, they need to be prepared to do it in the long run as well. Two of our moderators have stopped contributing; one of them was replaced. Many of our original candidates who were not elected have left the site as well (Maybe as moderators they would have stayed, but chances are they would have left). Candidates should be able to contribute to the site for some time to come. Future candidates: Please consider your future before running! Do you expect to go collage, get a job or have a family. How will this change your schedule?
If things change suddenly, you should also be prepared to cede you post. There isn't shame in resigning in order to pursue life, or be there for the people and tasks that mean the most to you.
Available - (esp. on chat)
Being able to reach people of authority informally can be helpful to many users. Sometimes using the formal channels of discussion can be intimidating or a tad unnecessary. In other cases it can set larger wheels in motion or create controversy, when an informal discussion can resolve the issue or dissuade unnecessary action. Its also nice to be able to chat to the boss!
Experienced & Valued Contributor
Moderators, armed with super powered close and open votes, as well as swaying opinions, have to understand site policy. One of the best ways to know what is current is to be actively involved in the site, answering and asking, to see when rules should be bent, and when not.
A contributor's post's "Value" is easily quantified here on stack exchange with reputation. So can experience. Moderators should have a fair amount of it. That is simple.
There is however an important aspect to observe here: Current value. Old posts, that are early to the site, can sometimes belie actual skill. The simplest and most basic answers on sites such as stack overflow can have some of the most votes. Be sure the investigate the users profile, and see where their reputation comes from. In the brand new (and quite nice) profiles, under the activity tab, there is a graph of the reputation that user has earned. Frequent plateaus reveal periodic absences and a steady decline in gain shows decreasing interest. They should also be present in the top few rows of the week, month and quarter tabs. This also applies for the edits and voters tab.
Active Cleanup - Review Queues, Tag Wikis, Flags and Edits
A moderator should be constantly using their power to clean up the site that is there first an primary purpose. Their contributions in this area can be measured through badges, and review queue stats. They should have a considerable number of edits and help full flags (visible in the activity tab of the profile)
Meta master - On Meta.SE!
Its obvious a moderator should be active on local meta, but also on Meta.SE as well. The major policies of stack exchange are determined there, and moderators should be actively trying to help the network as well as their site. Furthermore, many improvements to local sites can and are gained through Meta.SE as network wide changes.
Above and beyond
There are boxes of service for moderators to check. There are many that are optional, and some that aren't on the list. Moderators should check all of these. Nobody requires a moderator to advocate for our community or spread the word, but they still should go that extra mile, or at the very least, convince others to do so.
A diamond isn't a verified seal of a quality contributor. Moderators usually lose their diamonds for fairly major offensives or gross negligence. I've seen moderators trashing valid area 51 proposals (Down votes on highly voted example questions, etc) that might steal traffic and a bit of scope for there sites. I've seen moderators only check the bare boxes of there posts instead of handing it off to far more dedicated and worthy individuals. Many of these folks are still moderators. Use the criteria in this post, other posts and stackexchange blogs/official statements as a guide, before blindly voting for someone just because they are moderator on another site. Moderators are hard to replace, so choose wisely.
Chat is more important then you think! You may think its a waste of time, or just a social party, and to some extent it is. It is also the highly influential power user hangout, and can influence meta greatly (More then you would think: imagine meta posts to be press releases, while chat is the board room. That can often be the case [Okay literal folks, this is a bit exaggerated.]) It is also responsible for anomalies on the main site. Chat posts can determine the fate of a question, or influence a decision. Moderators should be active and aware of what happens in chat.
Moderators - Figureheads of Policy
Moderators are the figurehead of our site, to other sites and new users. Vote for who you think best represents the positive attributes of our site, and will work to keep them that way.