When you first start writing, before you realize that commercial fiction is something entirely different from expressing yourself in an artistic way, you might fool yourself into thinking that you have the most interesting and unique plot in the world and that no other author has ever possibly done this before. I am here to squash that dream with my hammer of cynicism.
Writing fantasy? Here's your plot: Young youth and/or adult (gender interchangeable) goes off on quest to save home and has awesome adventures with a spunky elf, a grumpy goblin, and a dour dwarf. There will be swords, rings, magicians, dragons, and a powerful enemy that will need to be defeated with the power of good. Youth will have moments insecurity and the entire story will be a bildungsroman.
Writing a crime drama? Here: Cranky criminal defense lawyer gets involved in crazy crime ring and will look to hardened street cop for advice and help. People will die. Cars might explode. Mafia ties will be exposed. A cover-up will be foiled. Corporate powerhouses will be taken down. There might be sex.
Speaking of sex, here's a romance novel: Heroine hates hero. Hero hates heroine. They have sex to spite each other. From their union blossoms love. A disgruntled mistress will enter the picture and try to spoil everything. A huge fight happens between the heroine and hero and they will temporarily depart. Things work out and then end up happily ever after.
Mystery? Boom! A body is discovered by adventurous teens/adults/senior citizens. Or something is found/stolen under mysterious circumstances. The adventurous (insert title here) work to solve the mystery while skittering bumbling city cops and merciless killers and/or thieves mess things up and make things difficult. Watch out for a twist ending!
Horror? Bam! Anything by Stephen King.
So on and so forth.
I'm just a horrible human being, aren't I? But if you look past the snark, there is a spark a truth. Most plots have been done before, and they've been done and well and badly depending on who's done the writing. It's not something you can really escape.
Now, before you go throw your computer out the window because of annoyance at me or because a sudden cloud of despair has blanketed your heart and made writing an impossibility, there is a ray of hope. You can still be unique in your writing. So go get your computers back out from the lawn and put the pitchforks away.
Uniqueness in writing, in my opinion, doesn't come from your plot so much as it comes from what you do with your plot. With literary fiction or non-genre fiction, things are a bit easier because you don't really have a formula that you're expected to follow. For example, with my new manuscript, it's not really a genre piece. There are elements of mystery and of romance, but it's not part of either of those genres. If you were going to call it anything, I suppose you could call it a slice of life, but even then, it's not that entirely. It's more an experiment of what I can do with words, and that leaves me free to do some things that I might not be able to do if it's for a genre. But with genre pieces, like a romance, there are certain points you need to hit and if you don't hit them, you get knocked for it.
How you hit the marks is what really makes you stand out with your writing. And how you write also makes you stand out. Your voice, your tone, the atmospheric pressure of your writing, it all works together to suck a reader in.
Interesting characters are a must too. You can have a really interesting plot (crime boss turned soccer mom discovers the coach is a hit-man sent out to assassinate a local politician for reasons unknown and now must rally her abandoned mafia ties in order to stop the hit from happening) ((I am so writing that novel)) but if your characters are wooden or feel like they're following a script, then your readers are going to ignore your writing in favor of something else. Having a unique character can go a long way in keeping your reader happy. Crime boss turned soccer mom can fall flat if she's stilted, or questions herself too much, or is just plain boring. She should have some kind of quirk, some kind of idiosyncrasy that sets her apart from all the other crime bosses. Why did she abandon the mob? Was it for her children? Was she tired? What's up with her? What are her motivations? And no, backstory doesn't count.
In closing, a book is like room. Your infrastructure is your genre of choice. It's the bare bones of your writing, what you have to include. The characters, the twists, the dialogue, the everything else, are the details, the decorations. How you mix and match is up to you. It you want crystal door knobs, that's your choice. Persian rugs? Sure! As long as everything works together, you're all set.