Chris Baty's No Plot? No Problem! (which is specifically about NaNoWriMo) has a piece of advice about what to write about:
Make two lists, one of stuff you like to read, and one of stuff you don't like to read. Write about the first list, and be careful to exclude the second list, which will do its very best to creep in on the grounds that since you don't like it, it must be Good For You!
The second part of that is rather more useful than the first. Inspiration can come up with things I really, really, really hate, and I have to learn to excise them quickly because the inspiration will run dry really quickly, and I will sit there wondering why if I don't recognize the root.
But it's really easy to like to read something and have no knack at all for writing it. I read stuff besides SF and fantasy, but I don't write anything else. I like the epistolary POV, but I don't have stories that would work in it. I like when intangible things are treated as tangible -- when Grendel assures Amelia in the Fugitives of Chaos that his mother keeps her blessings in a jar, and the Wine of the Gods in One for the Morning Glory includes such things as autumn, and a box in Jonathan Strange And Mr Norrell is neither blue nor lavender but heartache -- but while I have recently had an idea that might use it, heretofore that idea has been adamantly uninspiring.