And this is exactly what bothers me about fantasy. As far as fictional genres go, fantasy should have the most potential for originality of all. Because it doesn't have to portray the real world or be bound by its rules, sure, some fantasy stories do take place on Earth, but even those get room for making their own things up, and a lot of fantasy takes place in a world that is not real, giving the writers freedom to do their own thing. Now, you could argue that science-fiction also offers a lot of such freedom, and despite it's name, it's not like it has to be bound by real world science. But generally, science-fiction tries to convince audiences that it's about things that could happen in the future and/or some distant part of the universe. Thus, while science-fiction might give us technology that isn't possible with modern science, possibly with some sort of techno-babble explanation, it's generally not going to consciously rewrite the very structure of the universe, yet fantasy has this freedom, and some settings take it. So why is it that science-fiction authors can create worlds that truly feel alien, but fantasy authors rarely succeed in doing so? This annoys me to great ends.
The very fact that the mythical creatures aren't original prevents it from feeling original, considering that you actually see them regularly, and thus they have a major effect on the feel of the world, the whole messed up cosmology on the other hand which is fairly unique is (and despite all I have been saying about originality, in this case, I must say, thankfully.) something you rarely here about unless you obsessively read every bit of lore in the series.
The problem writers face with fantasy is that if you don't use some familiar tropes, it tends to distance the readers from the story because it's "too weird". The same problem arises in science fiction. You can make a world as alien as possible, but if you don't put enough familiar elements in there, it really turns most people off. This has come from many many discussions by writers at various SF/Fantasy conventions I've attended. Fantasy as a genre is actually highly traditional in a sense, and it relies heavily as a rule on folklore and mythology from the past. You could write something that's completely out there, but the typical reaction to your work would be, "I don't get it." Sad but true. I found this out the hard way. I tried writing a fantasy story once that didn't rely on any common fantasy tropes whatsoever and created a completely alien fantasy world, and the reaction to my work was...dead cold. Nobody was interested.
That's why fantasy games stick to the familiar, because it's safe. I think Morrowind is one of the most unique fantasy settings out there, but I've heard a lot of other people complain that it was "too weird", and so Beth decided to go back to a nice safe high-medieval fantasy setting for Oblivion. Then they released Shivering Isles, and I was appeased for my desire for original content, but again, other people complained that it was "too weird". I really think Bethsada ought to stop listening to those people and just make the world they envisioned. I really want to see the Black Marsh in one of the next games.