Does OS X enforce a maximum filename length or character restriction? What is the maximum filename character length of files in OS X?

Of secondary importance is whether some characters are allowed/disallowed in addition to any overall length restrictions.
From the font of all knowledge, the maximum file name size in the default HFS+ file system is 255 UTF-16 encoding units, so basically 255 characters. Technically, any unicode character can be used, but this may be practically limited by the application you are using - for example, the / character is used to delineate directories in pathnames, so you may not be allowed to name a single file stuff/things.txt, as this means "the file things.txt in the stuff directory" to the operating system. There are ways of "escaping" characters like / in filenames, but for general use it's discouraged.