If you make rules to address every situation that comes up, you'll be making rules forever (because users will keep coming up with new ideas) and then you'll end up with a large, unmanageable pile of not-necessarily-self-consistent rules, and an obligation to administer them. Yuck. That's no fun.
Instead, you want fewer rules and broader license. The CS department at my alma mater (Carnegie Mellon) had a pretty strong community when I was there -- there was an active lounge (students, staff, and faculty), there were departmental activities, and there was even a chore rotation for everything from cleaning the lounge fridge to maintaining the software to manage the monthly cheese buy (I am not making this up). This community had one rule, the Reasonable Person Principle: strive to be reasonable, grant that everyone else is doing the same thing, and don't take disagreement personally.
That all sounds fine and dandy, but what if definitions of "reasonable" collide? The RPP states:
Not all people share the same model of reasonableness, so disagreements inevitably occur. Under the reasonable person principle, the first thing to do is work it out privately (perhaps in person, since e-mail is known to amplify feelings). Indeed, many people would find it unreasonable to bring in third parties before trying personal discussion.
This is easier to do in a small, local community where you can sit down with the other person over coffee, but the idea still works even if the other person is halfway across the world. If you can at all manage to have a private synchronous conversation (phone, IM, whatever), instead of bouncing email back and forth, I've found that works better. If someone has hours to stew over, and work up a head of steam in replying to, a piece of email, things degenerate more quickly.
The CS department didn't have moderators (people with the authority to enforce rules against people), but if it had had moderators we would still have encouraged the community to address these problems where possible. You don't like that Joe cooks smelly curry in the microwave, or that user666 posts snarky comments on every post? Take some ownership and try to work it out amongst yourselves before asking a moderator to step in; it's what reasonable people do. If a problem comes to you as a moderator (or you, as a mod, are the person who objects to the behavior), then follow the RPP like anybody else. A conversation between people who have different perspectives on a community they care deeply about will accomplish way more than citing rule 42(a)(iii)(paragraph 17) (and adding paragraph 18, because who would have thought to regulate washroom viewings of the World Cup?).