People asked what properties are required for schema.org vocabularies but (afaik) there was no official response. Here is what I discovered from the Google Rich Snippets Testing Tool (← link to the test file) the closest thing we have advice on implementing vocabularies schema.org. Note that the warnings of the tool may or may not be consistent with schema.org vocabularies because none of this is really defined (sigh). You can also try to refer to the documentation Rich Snippets or on the mailing list .
you must include at least one property of the vocabulary that you use
Recipe Person and Product Require Property name
The event requires the properties name and dtstart (and that the url property of the event is on the same domain as the base URL)
MusicRecording requires name, url, duration and inAlbum properties
values are sometimes checked: for example, Event startdate and enddate plus MusicRecording time are validated against ISO 8601, but the cooking time and total Recipe time are not validated.
sometimes the tool gives incorrect warnings, for example a warning on the "dtstart" and "dtend" of Event (not startdate and enddate)
Few properties are marked as required. This means that if you go from