Yet, of course, the setting which are the most memorable are exactly those which succeed at doing that, and present a world that truly is unique! Morrowind, Geneforge/Avernum*, etc are outlasting their age much more than I expect, say, Skyrim to do.
Looking at these games, I think the key is to have the character
(rather than just the player) be new to the setting. The PC in Morrowind comes from one of the other provinces, and in fact that's a pretty major part of the game. In Avernum (Escape from the Pit, at least, it's the only one I've played so far but sequels set in the same place don't need to worry about describing the world so much as a lot of players have come from the first game) you're thrown into a deep cave system, and were expecting to die immediately, but have found a completely unexpected civilisation. In Geneforge (again, I've only played the first game so far) your character knows more lore, but a lot of it doesn't apply in the location the game's set on anyway.
The reason, perhaps, is that then you're able to find out about the world without breaking your suspension of disbelief, as your character would be doing exactly the same thing.
Well, the main thing is, again, when you talk fantasy, everyone knows the basics. Everyone knows what an Elf is (Pointy-eared bastards who live a very long time, tending to be attempting to live in harmony with nature.), what a Dwarf is (Usually drunk on ale, long beards, short stout people, live in mountains, mine and work with metals...), what Orcs and Goblins are (And maybe even Gnomes, Kender too if they're unlucky). I don't have to make someone new to the setting, I can simply start you off with 'Hey, our village is being raided by a bunch of Orcs, go kill!', and from there I can move the story forward and introduce extra stuff without worrying about overloading you with details. And even when I throw something odd at you, you can handwave a lack of knowledge from being from a village that doesn't see a race or a certain thing a lot.
Sci-Fi on the other hand, doesn't have traditional races that I've seen. I mean, you can completely stick with humans only, but that's limiting. You also have to handle introducing technology that's different, how things work in the setting (at least to a degree). It's a lot more work considering there aren't that many common elements that you don't have to explain, at least compared to Fantasy.