^Lack of resources, and many states competing for the same, was certainly the key driving force for colonization... That said, there are/were resources of note, including copper, silver and gold mines, along with marble and wood (for ships). Food sources were more scarce - even Athens had to import grain from the Crimea, through a co-operation between greek colonies there and two native peoples; it even used Scythian archers as a police force when the athenian army was away in the Peloponnesian war).
Farmland is there mostly in Thessaly and (to a smaller extent) Macedonia. Thessaly is actually pretty flat, and certainly when compared to the rest of the country; this is also why it had a reputation for its horse breeding and horse-riding military units.
Modern-day resources are another story, though, cause there are rare earths, natural gas, possibly oil, gold and minerals used for steel production.
And to tie this back to the ancient era: marble is still a lucrative trading resource, with high quality marble particularly from the island of Paros being a monopoly.