I'm pretty sure Andre Norton's used the "ancientEvilSpiritThingsofDOOOOOM" plot device...
Heh...and I figure if she could get away with not explaining any new technology, so can I. If you baffle people with description, they aren't as likely to ask how something works. ;-)
My current story kinda mixes elf-like folks with some telepathic abilities and a somewhat Firefly like universe. My brother accused me of stealing large portions of my main guy's speech and general attitude from Firefly, but since I started writing this well before I knew Firefly existed, that isn't true.
No magic in my story per se, but it might as well be.
I think of this story as "Harry Potter in space". Only that's a completely lousy description since there are no kids, no Voldemort, and no Hogwarts. But it is sort of like what would happen in the HP universe in five hundred years, if the wizards got their acts together.
And the roots of this story started out in my fanfiction days anyway so that's where the feeling comes from. Stylistically and plot wise I think it's closer to a Heinlein juvenile, or Lois McMaster Bujold's "Miles Vorkosigan" novels.
Oh, and as to bothers - even if you are stealing from Firefly, Firefly steals from other places. It's not plagiarism, it's an homage.