When I was judging the Pitch to Publication contest in July, one topic that continually came up was originality, and how it relates to what agents and publishers are looking for. No story is entirely original, but if you come up with something that's essentially a cut-out of another book, you'll struggle to sell it. On the flip side, something off-the-wall weird and creative might struggle to find an audience. One thing's for certain: it's almost impossible to find a book that's entirely free of tropes, because they're embedded in virtually all the stories we run into -- books, TV, and film. Even story structure itself can be based on tropes -- see the hero's journey, for example.
What's the difference between a trope and a cliche? Well, tropes are popular for a reason: they're themes and ideas constantly repeated because they resonate with people. Cliches, however, are ideas that have been overused to the point of inducing eye-rolling. But definitions can vary for different people. Love triangles, for example, are a trope, but are often called a cliche because they've been used in so many popular YA novels in the past few years. Personally, I'll never write one, because my books tend to be more mystery-and-plot-focused than romance-centric, but this is just my writing style. Some of my favourite series -- the Iron Fey by Julie Kagawa and The Infernal Devices by Cassandra Clare, for example -- use this trope. It can definitely work. Check out TV Tropes for an extensive list, and you'll find that almost everything in popular fiction is a trope.
So where does that leave originality?
Honestly? It's as subjective as anything else. I tend to look for originality in worldbuilding, for example, but there are plenty of successful novels that use really familiar ideas in their worldbuilding --like elemental magic, for example -- and that works for them. As for love triangles, for all the complaints... just look at the Amazon charts.
With that having been said, when I was judging the Pitch to Publication contest, I was looking for entries strong enough to pitch to agents. Often -- but not always -- the use of a too-familiar idea can indicate inexperience, and the lack of a strong authorial voice. A fresh, unique voice can breathe life into an overdone premise. Look at what Laini Taylor did with the angels/demons trope. Or Susan Ee with Angelfall -- mixing two popular tropes, angels and the apocalypse. And it's done extremely well. You need to hit that perfect mix of familiar and unfamiliar, but even that can vary between agents and publishers, and can be dependent on trends.
I try not to stress too much about it in my own writing. It's difficult. On the one hand, Darkness Watching had a lot of praise for being a different take on demons... but many readers didn't like it for the same reasons and all but said they'd have preferred a straight-up paranormal romance. Some readers actually prefer it if the tropes are familiar -- like those who want to know upfront if a romance ends happily, for example. I confess I like the element of surprise a little too much, and it was a little jarring to find that I'd actually have more success marketing my books if I focused on the familiar elements rather than the cool, original parts. As a reader, I go actively looking for books that try to do something different, but ultimately, it's the familiar-but-slightly-different that sells. The first line of the Amazon blurb of Darkness Watching says, "Think Buffy Meets Supernatural." Since I added that line just before a Bookbub ad, sales have vastly improved. It's a little difficult to ignore!
On the other hand, there's infinite scope for twists on familiar ideas, because every writer's voice is unique. Give five people the same writing prompt and you'll get five completely different stories. As I said, voice is key -- and possibly the most important part of winning over your audience.