Policies should be created when there's a need for policies. They form a manual of style for pages so people know how to write. They make sure that vandalism, spam, and trolling are known to be unacceptable. They ensure that people are polite and respectful to one another. There are all sorts of policies that can be created, but it's important to remember a few things before you set about creating them.
A policy should accomplish something
Policies should be clear and concise overviews that guide the editing process and the interactions of the community. To do that effectively, policies need to be practical solutions to actual problems. That's something communities often make the mistake of forgetting. One notable community, which will remain nameless, would often create policies for unimportant issues; you could not have more than 20 user-boxes on your user page, you could not have more than one video on your user page, and you had to have a category on your article or the entire article would be deleted.
Do those sound like real problems, and do their solutions seem necessary? They were all made based on the preferences of the admins rather than the need to fix real problems. That brings us to another very important point.
Keep It Simple, Stupid
No, you're not stupid, don’t worry! The KISS (Keep It Simple, Stupid) method to success is a way of making sure that your rules and things you want to teach people are as simplified and understandable as possible without losing their meaning or value. It's an incredibly valuable concept that should guide your policy creation and management.
In the case of those unnecessary policies that were already mentioned, one thing you want to avoid is "instruction creep." The idea of instruction creep has been around for awhile, often in corporate settings, and it's what happens when rules and regulations increase in size and quantity until they can no longer be readily understood or effectively enforced. This can hinder the success of a community, because it is directly contradictory to the KISS method. The primary way it hinders success is that users either can't comprehend that many policies, or they simply choose to ignore them when they see so many. Who wants to read 20 policies? - not just 20 rules, but sets of rules.
Let's paint a scenario that sometimes occurs. Say you have a lot of policies. What happens? Often, people don’t read them. That means they miss some of the most important ones, like manuals of style that teach them how to create and format pages. You then have a situation where people are creating lots of pages and edits that don't follow your guidelines. What is the usual reaction to this by administrators? Unfortunately, unless they've come to recognize that they have too many policies, it is likely to add more policies or begin blocking people - both of which go against what the community needs to succeed.
If you need to create policies, consider whether those policies are necessary. Look at the area of wiki editing or community interaction that the policy aims to cover. Is that area a problem? If not, then don't try to fix what isn't broken. If it is a problem, is the policy you're implementing a practical solution? If not, consider trimming it down. This leads to another important point.
Set realistic expectations for your community
If you expect people to quickly learn your policies, don't give them 20 new sets of rules to read right off the bat. That's not a realistic expectation to have of anyone. This is especially important for new communities. If you've just founded a community, then you don't need a lot of policies. That can be very harmful to the future of your community if you try to control everything at once, especially when there's nothing there to control yet. Let your community evolve naturally, and fill in policies when and if they are needed.
You also need to remember the nature of wikis, because they're not like other websites. A wiki is not a blog, it's not a social network, and it's not a message board. It's a little bit of everything all wrapped into one, so it's a very unique format. Because of that, it inherently has a learning curve. Not everyone is going to learn at the same pace, and that pace can very much be affected by how many policies a community has and how simple or complex those policies are. Some admins are fond of thinking "I learned fast, so you should too!" In reality, though, that's a meaningless statement. Not everyone will learn at your preferred speed, so don't expect them to. Everyone is different, and everyone should be given a chance to learn at their own pace so long as they're not disrupting the community. That's how new, inexperienced users become great editors.
In summary, keep things simple. Don't try to solve problems that don't exist. Avoid instruction creep. Everyone learns at their own pace, so make sure you're making things as easy as possible for people so long as it doesn't detract from the business of the community.