The Elder Scrolls, like most high fantasy games (including Fable) takes a lot from both actual history and culture, real mythologies (real in the sense that they are the mythologies of real civilizations.) most of the races for example don't look particularly original if you look down at their basis. The humans, of course, are all inspired by real life cultures and races, and the different types of elves are similar to certain elves that are common in other fantasy settings. The Khajiit and Argonians are based on a concept that often seems to come up when a work of speculative fiction, such as if a fantasy setting wants something different from the usual http://tvtropes.org/pmwiki/pmwiki.php/Main/FiveRaces, or a science-fiction setting where http://tvtropes.org/pmwiki/pmwiki.php/Main/RubberForeheadAliens aren't different enough from humans but the designers lack the creativity to come up with good http://tvtropes.org/pmwiki/pmwiki.php/Main/StarfishAliens (or alternately, they want the aliens to be major characters and they feel that it would be too hard for audiences to relate to completely alien lifeforms.) thus the concept is to take some animal of the creator's choice, some sort of feline or reptile being a popular choice, and make an anthropomorphic version, they walk on two legs, have two arms, speak human languages, but at the same time no one will actually confuse them for humans. So the races of the Elder Scrolls aren't exactly unique, but at the same time, Bethesda has tried to put a unique spin on them. Now, Khajiit and Argonians they can succeed in making pretty creative, and humans, as unlikely as it seems that a world with no connection to the real world would produce lifeforms that seem physically identical to real humans, have a justifiable reason to be in the world because they offer something that players can easily identify with, even orcs can be made relatively unique simply by the fact that orcs are rarely portrayed in games as being anything beyond always chaotic evil, but elves I think are beyond saving. The rest of the Elder Scrolls world is also filled with generic fantasy ideas that Bethesda tries to make less generic, how successful this is depends on the exact item in question. But I'd say that as a whole, the Elder Scrolls is more unique than most high fantasy games, but this says about as much as calling a movie better than Plan 9 From Outer Space.
Yes arena tipped over to the roman colosseum real world, and oblivion was quite alike to the generic medieval-fantasy world, but morrowind, that was completely unique, a truly alien world. Not sure about Redguard, Battlespire and Daggerfall though.
I wouldn't call it COMPLETELY unique, it had lots of creative ideas, certainly, but at the same time, it's not like Bethesda built it all from scratch. The Dunmeri culture pulls a lot from real world cultures, just cultures that are less commonly used in fantasy settings than Medieval Europe (or at least its theme park version.), the expansions brought in more unoriginal content by introducing goblins (in Tribunal) and various real animals, werewolves, and aspects of Nordic lore (in Bloodmoon.) And of course, adding giant mushrooms unto an otherwise fairly normal looking landscape doesn't make it completely unique. Regardless, Morrowind was pretty unique as far as high fantasy goes.