Wow, excellent question. I suppose the answer will vary quite a bit but I'm tempeted to use the definition of a "real bicycle" that Supreme Court Justice Potter Steward used for pornography; "I can't define it but I know it when I see it".

Objectively, I tend to consider that real bicycles come with Acera or above for MTB's and Sora or above for road bikes. The frame is also a factor. For example. forks with the dropouts made by just flattening the blade ends and slotting them are a disqualifier. It will be interesting to see other poster's thoughts.