What characters should be restricted from a Unix file name?

Consider a Save As dialog with a free text entry where the user enters a file name as free text, then clicks a Save button. The software then validates the file name, and saves the file if the name is valid.

On a Unix file system, what rules should be applied in the validation such that:

• The name will not be difficult to manipulate later in terms of escaping special characters, etc.
• The rules are not so restrictive that saving a file becomes non-user-friendly.

So basically, what is the minimum set of characters that should be restricted from a Unix file name?