|This is an essay. It expresses the opinions and ideas of some Wikimedians but may not have wide support. This is not policy on Meta, but it may be a policy or guideline on other Wikimedia projects. Feel free to update this page as needed, or use the discussion page to propose major changes.
Common sense is at once the most indispensible piece of policy within Wikipedia and the most difficult to pin down. Many other policies, such as Don't be a jerk, are really just restatements of "use common sense".
The problem is that no definition of common sense readily exists. The danger this leads to is the possibility that a user will confuse common sense with "the way they would do it." Such a confusion is antithetical to the methodology of Wikipedia, which relies precisely on the fact that people who see and do things totally differently from how you do are going to be editing your work.
The benefit is that common sense is uniquely able to deal with the fact of life that it is impossible to exhaustively list all of the things that are bad to do. Policy is not and cannot be an exhaustive list of the things that one should not do. Without some application of common sense, policy becomes a suicide pact.
Common sense is always controversial. This does not mean that it is wrong. It does, however, mean that measures have to be taken to ensure that common sense is both common and sensical.
In practical terms, this means that when you are doing something and justifying it with common sense, you should run it by some other people - especially people who have shown themselves to have the respect of the community. If they agree with you, then it is more likely that you're acting with common sense. One should be careful, in doing this, to avoid a situation where one could be accused of working within a cabal or echo chamber, or of treating w:groupthink as common sense. If you find that you are always asking the same group of people for advice, and yet are still sparking edit/block/delete/whatever wars and generating a lot of conflict, consider expanding the group of people you ask. It may be an especially good idea to ask new users as well as experienced users.
Given enough applications of common sense, eventually a situation will arise in which, despite asking a number of people, despite sincerely acting in good faith, and despite basically doing everything right, a large number of respected and sensible people will disagree with an action that is justified as common sense. This is because there are areas that it turns out that there is no consensus on, and because reasonable people are capable of disagreement.