Principle of Least Privilege
While there are many other parts of .NET security we haven't covered here, one more important point should be mentioned. The Microsoft platform has to become more security-aware. To this end, security policy should follow the principle of least privilege. Assemblies should only be granted the minimum set of privileges that are needed to run. This will minimize the chance that bugs or malicious code will cause damage. In other words, "If in doubt, leave it out." You can always relax security policy if necessary.