So, if they were so different then how could they identify themselves as Greek. There were several things that made them Greek. First, they spoke the same language. This is opposite of the situation in Mesopotamia where people spoke any one of dozen different languages. Second, they had a common set of gods. Though they worshipped differently, they shared the same holy places and myths.
The Greek City States did share three common political values; otherwise known as the 3 Cs, Constitution, Colonialism, and Competition.