1. Characters. They can have three heads, two noses or what have you, but the important point is I've got to be able to root for the characters, whether it is to cheer them on to success or hope they come crashing down. There has to be something about them I love or loathe but makes me want to read on to find out what happens to them.
2. A sense of how the world is governed. I don't need all the details, they'll get in the way of the story, but I need to know that your characters know the rules of their world and how these affect them as the story progresses. For example, in a world where there is no oxygen, what do your characters breathe instead? DO they breathe (or are they water dwellers)?
3. The details given are relevant to the story. Enough said I think!
4. A sense of what it would be like to live in that world. I don't necessarily have to want to live there. I may be very glad I don't in fact but this sense of what it would be like is enough for me to create my own mental images of what your fictional world might look like. That in turn helps me engage with that world and the characters you've put in it.
5. A sense that it could exist somewhere out in the universes. No matter how unlikely, the possibility should be there! This means that there has to be a sense of a world that can sustain itself, possibly trades with other worlds and so on.