You are all missing the point of the title. First of all, the title does not have the article the directly before Earth. It is "The Man from Earth". Nowhere does it say The Earth. I mention this because some people are debating this point in this thread.
As to the meaning of the title...I believe that it implies that the main character has been around so long, and lived in so many different parts of the world, that he cannot clearly define his existence by a single country, race, or language (i.e. French, German, Spaniard, Arabic, etc). Those countries and customs/languages did not exist 14,000 years ago (which is how old he is). Therefore, the most accurate description of his origins is that he's simply a man that existed on the planet all those years; hence the man from Earth. I think it's a great title.