When I vote on Mod applications, I have four basic criteria which I use to assess the applicant.
Professionalism/Integrity: This is the most important category for me - if someone fails this, then I automatically vote no to their application.
Suitability: Also another important category, for this I consider post count, infraction history, user attitude, maturity and whether the user makes a diverse contribution. In terms of post count, I am fairly rigid - I automatically eliminate anyone with under 1,000 posts because I dont think this shows enough of a sustained contribution. Just to note, I dont think having an infraction history automatically rules a person out from becoming a Moderator - if you have been infracted for going off-topic or something minor, I wouldnt really hold this against you. When I consider this, I look at the severity and the length of time which has expired since the last infraction.
Demand: I look at demand for a Moderator overall and in the specific forums the user has applied for Modship. To me, overall demand trumps forum specific demand because most of our moderating occurs in the general forums.
Commitment: An applicant must demonstrate to me that they are going to stay on once Modded. It irritates me when we appoint a person who promptly disappears from the site when they start Modding. Whilst you dont need to be super active, you have a responsibility to carry out certain duties as a Moderator and a failure to do so makes the job tougher for the rest of us.
The top two criteria are deal breakers for me, whereas I am willing to be more flexible on the last two (i.e. I would vote for a great applicant even if I felt that we didnt have the demand).