Some notes from an IRC conversation 20070118.
Regarding a possible abstract "mandatory" property:
Rather than abstracting to "mandatory" / "nice-to-have", I think it is worth seeing if such a property is in the 80/20 of those containing microformats first, and designing accordingly.
Creating abstract generic properties in a vacuum (or before / without constraining them with specific Best Practices for Examples Pages per the The microformats process) will likely result in them either being overly watered down, or overly complex in order to handle "possible cases" instead of "actual cases".
Thus abstract properties tend to be an instance of rejected-formats.