Just a quick note about world-building, and I’m pretty sure it applies equally well to science fiction and fantasy. (Not surprising, since science fiction is actually a subgenre within the fantasy genre.)
Whenever you introduce an element into your story that isn’t found in our own world, you need to consider carefully how it fits into the world you’re envisioning. Whether it’s nanobots or ghosts, what are the implications?
Let’s say you want to include a ghost in your story. Okay — does everybody become a ghost when they die? If so, won’t your world be littered with millions of ghosts drifting around? That’s a question without an easy solution. Do ghosts eat? If not, what source of energy do they use to travel around? Are ghosts subject to the laws of physics, such as gravity? I once critiqued a manuscript that included a ghost dog, and the author treated the dog’s interaction with physics very inconsistently. Sometimes it could walk through walls, but other times you had to open the door to let it out.
Turning to technology, let’s say your scientists have developed some new technique. It could be a brain-to-computer interface, a faster-than-light drive, a drug that produces telepathy, anything at all. No matter what it is, questions need to be asked. Is the technology widely available, or scarce? Is it expensive or cheap? Does everybody know about it, or is it a closely held secret? Can anybody get their hands on it, or is it restricted to the ruling class? What happens when criminals get hold of it? How will its existence or its use affect people — their minds, their lives, or the culture in which they live? What happens if it malfunctions?
Failing to consider these questions carefully is the mark of a weak, sloppy writer. To build a world, you need to fit the planks together with care, preferably so that the joins are not obvious. Sure, use glue, but a little dab’ll do ya. Use more only if you dare.