When we learn about classical civilizations in school, we're generally taught about Greek and Roman gods together, as if they're one set of deities with two different names. But really, why bother memorizing two sets of names at all? Why not just pick one and stick with it, and let the serious scholars deal with when to use Jupiter and when to use Zeus, just as they're the only ones who care about the difference between Clytemnestra and Klytaemnestra?
The truth is, they aren't as similar as we've been led to believe. They're as different as the musical Camelot is from T.H. White's The Once And Future King, and for the same reason: one story becomes two when told at different times, by different people, and for different reasons.
The Greeks had their gods first, before there was any such place as Rome and any such culture as the Romans. They built temples, held holidays, and told stories based on these gods, and enough of those buildings, documents, and accounts have survived to give us a certain idea of what those gods were like. Zeus the eternal philanderer, Hermes the mischief-maker, Athena the protector of soldiers: there's a certain consistency to be found in the gods viewed as characters. Athena is always good to have on your side, while Zeus's positive attention tends to be accompanied by at least as much negative attention from his wife, Hera, so king of the gods or no, it might be better not to catch his eye. You can get an idea of cultural values from examining the gods: Dionysos, god of wine, can be pleasant enough, but is in some ways the wildest and most threatening of the gods, to be treated with cautious respect - just like a strong brew!
While the Romans decided to import Greek gods, since Greece was considered a highly cultured area everyone should borrow as many ideas from as possible, they nevertheless had their own distinct culture with a different set of values. Roman ideals of duty, obedience, and reserve sat oddly on the willful Roman deities. Rather than change those values to suit the religion, they changed the religion to suit, indeed to support, the values. The warrior woman Athena, patron of the powerful city of Athens in Greece and by no means a god to be taken lightly, didn't suit the Roman idea of arms and war as a strictly masculine activity, and Minerva ceded a good deal of her importance to Mars, in whom it's difficult to see the reckless, ultimately undesirable Ares of the Iliad.
The Roman "versions" of Greek gods were, overall, a more socially acceptable lot, representing various virtues and performing associated functions without the tendency of their Greek counterparts to run around in plays behaving badly and losing divine dignity. No doubt this was to the benefit of the Roman state. But all the same, there tends to be something just a little flat about them. They make better social pillars, but as characters...there's something missing, something not quite tangible but real for all that. The Greek and Roman gods may have had the same symbols, clothing, and aspects, but they were not the same people.