My guesses:
1) Greek race and culture would have survived, just like the Egyptians, Babylonians and Jews. Persians were famous for their cultural tolerance.
2) Achaemenid Dynasty survives for a few more generations. It would collapse anyway, for it was unstable even without outside threats.
3) More Greek culture elements in Persian Empire, for the Persians borrowed a lot.
4) No Alexander's empire. Even the tolerant Persians would not allow a potential threat like the Macedonians to continue growing.
5) the Romans kept on expanding.
Ideas? What are the long term consequences?