Well, whatever other qualifications there may be, a ballad (in folk-type terms) has to tell a story. Greensvleeves doesn't qualify.

The Scottish and English traditional ballads have a number of typical characteristics, not all of which always apply to a particular ballad. For instance, a ballad, so considered, doesn't take sides, doesn't tell you what to think about the situation described--doesn't say, implicitly or explicitly, "Oh, isn't this terrible?" or "Oh, wasn't he a bastard? It not infrequently jumps, without warning, from one speaker to another, and expects you to figure out who is speaking by the context. It doesn't showcase the singer; it's the story that matters. Those are some characteristics that come immediately to mind. I'm sure others can name some I've forgotten at the moment.

Dave Oesterreich