Since city-states were independent and even made war on each other, and they spoke different dialects (I'm not sure how close different Greek dialects are, or if, say, an Ionian could understand a Dorian), where does this broader idea of Greek identity come from and what were the properties a city/community had to have to be considered Greek?
Ive read some material attached to this movement (http://macedonia-evidence.org/obama-letter.html) so ill attempt to answer this. This evidence is used specifically in reference to Alexander the Great but is generally applicable across the hellenistic world. Essentially it was a cultural/ethnic identity not a political bond. The main defining characteristics are shared religion, language, and participation in the olympics. Olympic participation is one of the best contemporary indication of "Greenness" due to the regulations for participating. I've visited olympia twice so this information is from a UCONN hellenic studies program director and confirmed by (http://www.olympia-greece.org/ancient-olympics1.html) One could not participate in the olympics unless they were considered Greek by the other city states, to the point of foreign bred horses being excluded.