The very first element, in my opinio, is world-building. The reader must feel himself transported into a fantastic world that one could plausibly, in his wildest imagination, imagine being able to work.
World -building is absolutely crucial. If this is not done well, I think, nothing else matters no one will read a series with a crappy world.
The Dune series is a brilliant example of world-building. A history of how that state of affairs came to be, so it seems "organic" not manufactured. Its a world I would like to visit; and a certain optimism seems to underpin it -- nuclear weapons have been banned in the "world" which is actually an empire of one million worlds.
One thing I like about the world of Dune is that the series does not depend on "aliens" to make things exotic and interesting. At the same time it explores the potential evolutionary variety of the human species. The Bene Tleixu are humans so strange they might as well be "aliens."
The other thing I would add here is that, having created such an elaborate world, you have to have a story BIG enough to fill it. I guess what I mean by that is that Dune wouldn't have worked if the story was too small for the world in which it was set; nothing should be wasted -- no element, no created technology, no astronomical phenomena, no characters, nothing.
Some might disagree with me here, but I think a smooth, rhythmic prose writing style is essential.