Moderator votes are "binding" - meaning that, while it takes 5 close votes to close a question, a single close vote from moderator will do it no matter how many other people voted before.
I would propose that moderators try (as much as possible) and refrain from performing actions that are binding - especially voting to close a question - unless it's one of 3 exceptional cases:
There's a VERY clear community consensus that the question ought to be closed (3+ existing close votes, many flags, negative comments, negative consensus on meta or chat).
The question is so 100% egregious that it must be closed immediately lest it cause severe immediate damage to the site (I'm having a hard time coming up with such emergency example, but let's say someone posts a clearly off-topic AND extremely offensive question - something you don't want to show up in Google search). Even in this case, I would posit that asking on META or chat first would be ideal, and waiting a reasonable (5 mins) time for people in community to pipe up.
The item was extensively publically discussed by the community (I'd propose the threshold of at least 4-6 people, for at least 1-3 hours; not sure of best limits) on Meta or chat; and after that discussion there's a clear disagreement with nowhere near strong majority, never mind consensus. As far as I understand, this case is specifically where moderators were meant to interfere - ideally, when the community agrees to bring it to a moderator decision. However, I strongly feel that usually the moderator action happens too soon before there's a consensus.
Please note that the above does NOT cover 2 cases where I strongly believe a moderator vote is a bad idea:
Moderator votes to close because as private community member, he feels the question is worth closing. I know one of our mods recently voiced this sentiment. It's a bit unfair to mods as people, but if you're the first/second guy to vote to close, please wait till community opinion is more clearly expressed and/or some discussion takes place. With great power comes great responsibility, and all :)
Please note "well if the community wants it, they can vote to re-open" is an extremely annoying and very wrong cop-out. First, it's hard to find 5 people willing to bother to look into the issue, analyze, agree, AND vote to open, all in reasonable amount of time. Second, a lot of people would not vote to re-open seeing a moderator name on the closing even if they would have otherwise (I've seen that happen).
The question MAY be bad, and is controversial, and there's an active discussion.
What would be a good action to take would be to lock/protect the question to avoid the discussion taking place on main site (if one is happening, or undesirable answers appear), and forcefully suggest to people to "take it outside" - e.g. chat or Meta.
Now, if whoever wants to keep the question refuses to do that, this is a proper ground to close the question.
DISCLAIMER1: This post is NOT a reflection of any specific problem with current SciFi.SE moderators, though at times they have exhibited the actions that I will suggest above as "should be avoided". But the opinions expressed here have gelled back when I was a newbie on StackOverflow and had absolutely zero interaction of my own with moderator practices - merely reading moderation rules and observing mods in action, and are not a reflection on any specific moderator - rather, on overall rules.
DISCLAIMER2: The timing of this post was affected by 3 things (none of which was a specific action by a moderator):
Gilles' post in chat yesterday indicating that moderators should not set policy and only get involved when the community can't come to a consensus (did I get that right?)
the upcoming moderator elections. I want to have the policy discussed and set BEFORE we have new moderators (and can have input from current ones); AND allow voters who care about this issue to be able to gauge moderator candidate's views on the topic.
Just came to the front of my "questions to ask on meta" buffer queue :)