I'm starting to get annoyed by the word mash-up. It's being used too widely and at the expense of other, better descriptors. It's become a buzzword, which means that it's nearly meaningless.
I wouldn't have this problem if the connotations of mash-up were more positive. For me, mash-up means "goofy, silly, poorly thought-out, capitalizing on a trend". The first use of mash-up I ran into was in reference to Pride and Prejudice and Zombies—"a Jane Austen-zombie mash-up". I liked PP+Z. It had a good hook, and pretty decent execution. But then everyone jumped on the bandwagon and within months we were inundated with titles. Some of those books, or at least the hooks that pitched them, sounded dubious. "How," I asked myself, "is Little Women improved with werewolves, or Huck Finn with zombies?" The reviews I've read seem to confirm this.*
So mash-up went from describing a blend of apparently incompatible ideas to describing a blend of apparently incompatible ideas that doesn't succeed. The core meaning's still there, of course, but when a good mash-up shows up or a book/movie/game/show is pitched as one, I think people are more likely to write it off. "It's a mash-up," they say, "so it can't be any good, really." (Possibly this is what I'm doing with Little Women and Werewolves, et al.)
And now on to what sparked this post: I've seen Cowboys and Aliens described as a mash-up, and I don't think it is. Cowboys and aliens are not incompatible ideas the way the way regency romance and the living dead are. It's fairly easy to imagine cowboys and aliens interacting, after all, whereas a large part of the hook for PP+Z was how they were going to pull it off. If Cowboys and Aliens is a mash-up, so is Midsummer Night's Dream, which is essentially a romantic comedy with fairies. If Dream were a movie today, yes, I'd go watch it because hey, fairies, but I'd go in expecting it to be kind of bad and the fairies to be not entirely necessary to the story. I'm certainly going to go into Cowboys and Aliens expecting to be entertained, but nothing more—not because it's a Hollywood summer blockbuster, but because it's been pitched as a mash-up.
So what are the other, better descriptors I mentioned? How can we describe fiction that blends ideas without resorting to the shorthand du jour? Option one is to pitch it as X meets Y—the OK Corral meets Independence Day, romantic comedy meets English folklore. Option two is to form a compound noun—alien invasion western, fantasy romcom. I'll also throw out blend, mix, and combination as possibilities. Firefly is not a cowboys-space mash-up, it's a show about the Old West in space, or a space western. Calling it a mash-up would do it a disservice, and the same goes for a lot of the other work that's getting labelled as such. I think more people would pick up those books or watch those movies if mash-up never featured in the blurbs.
My suggestion, then: If we're going to use mash-up to describe anything, let's use it for the creative works with apparently incompatible ideas—Julius Caesar and unicorns, to throw an idea out there, or Adam and Eve and pirates. Anything we can imagine scenarios for, and anything that can be described without using mash-up should be. The word is perfectly functional, but the more functions we give it, the more it's used when it doesn't have to be, the less it's going to mean, and what use is a descriptive word without meaning?
Interested in mash-ups? The Qwillery has a thorough list of them.
* True story: I work at a bookstore and we can't move Sense and Sensibility and Sea Monsters for anything.