Acropolis of Athens by Leo von Klenze
Note: I am neither an historian nor an anthropologist. My understanding of ancient Greece does not approach the scholarly level; this post is instead brainstorming what a D&D setting emulating this culture and era would be like.
A few weeks ago on the Wednesday livechat on Tenkar's Tavern, I broached the subject of deriving inspiration from eras and settings aside from the standard pseudo-Medieval Tolkienish mode. I figured that the cultures and myths of ancient Greece had great potential, in no small part due to the presence of several iconic monsters as well as the emphasis on exploration, divine meddling, and the exploits of legendary heroes. The tales of Jason and the Golden Fleece are not unlike a party of adventurers embarking on a quest to find a unique treasure. Odysseus' journey back home is full of strange lands teeming with dangerous hazards and monsters standing between him and his beloved family back in Ithaca.
The following ideas and suggestions below are designed as a springboard of ideas for gamers seeking to make a fantasy counterpart ancient Greece in their original setting rather than "ancient Greece, but with dragons and magic-users." This of how Al-Qadim is a fantastic setting counterpart of ancient Arabia, or how Spears of the Dawn is an original setting borrowing elements from various medieval African nations.
In traditional D&D, there are multiple gods and goddesses with dominion over various aspects of reality. Most people are free to worship multiple deities, but clerics are generally restricted to the worship of one whether in terms of general setting expectations or grounded in the game mechanics. There's also the fact that clerics had to closely match their deity's moral ideology in order to gain spells from them. In this sense, most D&D clerics are monolatrists, who recognize the existence of multiple deities but only choose to worship one.
In a fantasy counterpart Greek game, clerics, druids, and religious folk in general would make prayers and sacrifices to deities based upon the circumstances of the time. A cleric invoking a healing spell might be calling upon the god of healing and mercy, while the province of battle-spells such as spiritual weapon or shield of faith might fall under the war god's dominion. In Labyrinth Lord and most OSR games, clerics are unrestricted magic-wise with the exception of spells opposing their alignment, while in Pathfinder one's patron deity determines your domains and code of conduct.
For Pathfinder, there are several alternatives for a genuinely polytheist cleric. The 3.0 sourcebook Deities & Demigods allowed someone to worship the pantheon as a whole, picking two domains of their choice. The Midgard Campaign Setting allowed clerics to cycle through deities of their pantheon on a 2-week basis, switching out codes of conducts and domains for the particular god or goddess they were closest to at the time. For OSR games, it might be a good thematic choice for clerics to be able to use edged weapons (the blunt-only restriction comes from a Christian prohibition on priests from shedding blood).
The gods and goddesses of a fantasy counterpart Greek game will be largely amoral beings; they are fallible like mortals and prone to vice. Like the Judgment of Paris, a notable or influential mortal who helps or favors one deity over another might invoke the latter's petty wrath. Worshipers and divine spellcasters might recognize that their deities might be cruel at times, but they are responsible for the maintenance of reality and can be merciful and generous to those who are loyal. For this reason, it might be best to separate deities from the alignment system and allow virtuous and vile folk alike to receive their aid (and their wrath).
An important thing to consider is that unless you want a "rage against the heavens" game, you should downplay some of the more evil and wicked aspects of deities ancient Greek deities. Not just to discourage PCs from seeking their aid, but to also avoid the GM coming off as a bully with nigh-invincible NPCs who will punish you irregardless or making them irredeemable to the point that the players don't want to play in the setting. In Greek mythology, Zeus was a serial rapist and Aphrodite made a mortal woman's life hell for simply being born the most beautiful woman in the land (a circumstance of birth no fault of her own). With the exception of deities you wish to be blatantly antagonistic and evil, it might be best to have gods to mostly visit their wrath on more legitimate targets, such as mortals who openly curse their names, those who try to misuse their divine gifts, and those who steal from sacred sites and temples.
Another way to grant one's setting a more authentic vibe is the introduction of Mystery Cults. For those not satisfied with the dominant culture's pantheon, there are secret societies who only revealed their true beliefs and rites to a select few initiates. They may be dedicated to the worship of wicked and foul beings who promise power and privilege in the afterlife in exchange for inflicting misery on others; they might practice beliefs considered heretical by the orthodox temples, or members of a persecuted faith, social class, or culture who wish to find liberation or continue their teachings in secret; or they might be a front for secret bacchanal gatherings of excess and behavior frowned upon by polite society.
Even though people may belong to a greater region with a shared language and culture, ancient Greece saw the rise of the polis, or city-state, as the dominant political power. There were about a hundred poleis (plural form of polis) and more or less self-governing for much of their history. Due to the nation's mountainous terrain, there was not as much physical contact between the city-states. This resulted in the formation of unique variations on dialects and cultural identities. Additionally, certain deities were strongly associated with particular city-states, most notably Athens and Athena.
In a fantasy counterpart Greek setting, the nation as it exists might be understood more as a sense of a shared cultural identity of one people holding the same popular religion and language, but little in the way of an official body of government. People are more likely to identify primarily with the polis or region they were brought up in, and territorial expansions by neighbors would be fought back as fiercely as if it was an invading army from a foreign culture.
Using the advantages of themed cities and the impossibilities of a fantasy world, making each polis unique can really add to the sense of setting flair and why the realm is far from unified. One polis might be a militaristic fortress where every citizen is trained in the use of weapons; another might have installed the radical concept of democracy after the overthrow of a corrupt autocracy; another might be dominated by a non-human culture such as centaurs or merfolk; and another might be a hidden city, protected by a conclave of illusionists working together to generate the image of ruined streets and titanic beasts to ward off invaders and explorers!
Travel between poleis would be difficult, in no small part due to the many mountainous valleys and island chains. While small villages and folk living outside the great cities definitely exist, the seat of government would most undoubtedly exist within the confines of urban centers.
Unlike in medieval Europe, magic and religion were not regarded as entirely separate things. Hecate was the patron goddess of magic, and myths were replete with sorceresses such as Circe and magical items such as a bag holding the four winds inside its skin. The ancient Greeks regarded magic as a means and end to power, and its practice tended to be a more private affair in comparison to the divine rituals and sacrifices meant to give the gods their due.
In a fantasy counterpart Greek game, magic would be viewed as an amoral force (neither good nor evil), a way of finding out the secrets of the world to gain power over reality and make the forces of nature do one's bidding. Some common magic items might include curse tablets, written inscriptions and condemnations asking the gods to do harm to one's enemies. Amulets are created, bought, and sold to guard against such curses as well as the evil eye; as a result, they are one of the most widespread forms of magic items. The material and otherworldly forces are connected, and spells require rites and formula to perform much like material components. Magic which is considered exalted or closer to the gods is known as theurgy (high magic); practitioners of this art look down on those who use magic in a short-sighted, fraudulent and/or superstitious way (goetic, or low magic).
Game-wise, the difference between high and low magic is not in the classes themselves but rather the kind of spells cast. Harmful and illusion-based magic would be regarded as 'low magic,' while divination would be seen as 'high magic' regardless of whether it comes from a cleric or magic-user. Illusionists and magic-users might alternately be pious (recognizing the power deities grant to reality) and irreligious (only caring about their granted powers for personal ends).
Although dwarves, elves, and halflings are a classic D&D staple, there exists iconic Greek monsters with humanoid features such as the centaur, cyclops, gorgon, harpy, minotaur, and satyr. Although several of them traditionally existed as creatures hostile to humanity or possess features unbalanced for gameplay purposes, the original dwarves and elves also had super-powerful abilities and were distrustful of mortals as well. It can be a fun touch to not only have societies and tribes of mythological humanoids for the party to meet during their adventures, but to play as them too! As detailed below under "Shameless Plugs," there's quite a bit of existing material for monster PCs of Pathfinder and OSR retroclones out there already. But for this part, we can focus on how said creatures would fit into a fantasy counterpart Greek game.
Centaurs are forest-dwelling hedonists who are also strong warriors. Their relationship with humanity would be mixed; on the one hand, they scoff at the clustered city-states of lifeless stone and human culture's emphasis on moderation (an important virtue in ancient Greek society). On the other hand, they appreciate the fine wine, bronze and iron weapons, and other valuable materials said humans can give them in trade. Centaur settlements would be mobile, going from forest grove to valley in semi-nomadic patterns when one place runs low on resources.
Cyclopes are a primordial race of island-dwelling giants with strong ties to the god of the sea. They are incredibly skilled artisans and the greatest among their numbers once forged thunderbolts for the sky god to use as weapons. Cyclopes are staunch defenders of their islands from foreign ships, and as such are regarded as violent, savage brutes by humans and other seafaring races. Some tribes live deep underground, helping direct lava flows to help the god of the forge build impressive cities which result in volcanic eruptions above-ground.
Dryads, Faun, Nymphs, and Satyrs are benevolent nature spirits tied to a specific piece of land. Many humans and mortals worship them as deities, and while far below the power level of the pantheon, they are capable as a group of granting good fortune and divine spells to their worshipers. They often possess forms similar to humans, but with noticeable plant or animal features. They might be either helpful or hostile to mortal travelers depending on how they treat the surrounding environment.
Gorgons are transformed initiates belonging to serpent cults. They possess great powers, able to use their legendary gaze to turn their enemies to stone or grant supernatural insight to their allies. Depending on the polis and the deities favored there, gorgons might either be feared yet revered religious figures responsible for leading rituals and festivals, or hated monsters believed to make pacts with forbidden powers.
Harpies are winged folk, most of them women, who are born as the result of a pregnant mother eating food grown in a witch's garden. Although they tend to not have their own culture as a result of this, harpies' aerial talents allow them many opportunities to find others of their kind and move to more tolerant locales. They flock to poleis which are centers of art and learning, for harpies have an uncanny gift for singing, persuasion, and theater. Finally, they're highly sought-after as explorers and messengers, for the sea and mountainous terrain serves as minor impediments to those born with wings.
Minotaurs are believed to originate from what is now the labyrinthine ruins of an island-city. Very tall and possessed of a pair of bull-like horns, many wealthy families seek them as mercenaries and soldiers, an occupation the minotaurs are all too happy to accept. They are also very cunning in skirmish warfare and possess an innate sense of direction, make it nearly impossible for them to get lost in terrain already traversed. Minotaur settlements tend to be virtual mazes with few if any landmarks, making it impractical to assault or occupy by foreign enemies.
Inspirational Material/Shameless Plugs
For Pathfinder and Labyrinth Lord (OSR) games, I wrote up products for versions of monsters playable from level 1, including centaurs, dryads, giants, harpies, and medusa. In regards to other products, Barrel Rider Games has a few monster classes for Labyrinth Lord such as the Minotaur. For Pathfinder, Rite Publishing has a Level 1-20 monster class for the same kind of monster. Purple Duck Games' Legendary Races series includes a cyclops, harpy, and medusa.
Edit: My blog post got picked up by OSR Today (thanks for the mention, McCartan!), who recommended some more material for these kinds of games:
Many years ago, TSR released HR6: Age of Heroes Campaign Sourcebook [AL] for Advanced Dungeons & Dragons 2nd Edition. There have been other published products that handled the setting, including Mythic Greece: The Age of Heroes [AL], which was published for Rolemaster Classic [AL] and Fantasy Hero [AL] many years ago. Although these books are old, they contain a lot of great information for those interested in running a campaign set in this particular setting. Of course, there’s also Mazes & Minotaurs, which is a complete game created around the setting which can be used for play as well.