If we got a hundred Horror readers into a coffee house and a hundred Action readers into the coffee house next door then we asked each group what they like about their favorite stories, what we would discover?
That the horror fans all look for a certain experience from stories and the Action fans all look for a different experience from stories...but within each group there would be certain story experiences that everyone in that group look for. Same for Romance readers, Western readers, Erotica readers, Mystery readers, etc. Each group would name different elements that they expect to find in a good story.
These elements are the things that define the genre the story fits into. For example:
Romance - love, obstacles to love, a happy ending, good-looking characters, chemistry between characters, scenes of inner conflict
Mystery - an unsolved murder, a detective, detective attempts to solve case and also faces inner conflicts, solution to murder, scenes of investigation and discovery of secrets
Horror - danger to main characters, a threat that is paranormal or supernatural, scenes of tension, ending may be happy or unhappy
These things are called tropes. They are the accepted conventions for the genre you are writing in. Every genre has certain character types, specific scenes and definite elements that make it what it is. Without a love story, you don't have a Romance. Without ranchers and gunfights, you don't have a Western.
When you advertise your stories as being of a certain genre, you create an expectation in the mind of the readers even before they read the first page. They will be looking for certain things in your writing and you had better have those things in there or your readers will feel let down.
Does your blurb say your story is a zombie story Then you had better have plenty of shambling undead in there and characters fighting for survival.
Categorized yourself as Detective Fiction? Then make sure you have a detective and something to detect.
This may sound obvious but some writers go so far to avoid cliche that their work no longer fits within the genre it set out to be part of. And should all cliche be avoided anyway?
Tropes that are overused eventually become cliche. But that doesn't mean all cliche is bad in a reader's eyes.
How many horror movies tell the same story? And how many people flock to see them?
How many detective novels have the same basic plot? And how many of them are bestsellers every month?
The fact is, readers of genre fiction feel comfortable seeing some familiar elements over and over again. After all, it is because of these elements that they are fans of the genre in the first place. Putting a fresh twist on something and being original is great but remember that trying too hard to avoid cliche might make you end up writing something that is unrecognizabe to readers as belonging to the genre they want to read.
Yes, avoid being too cliche in your story but remember to give the readers what they want. What they expect.
The top grossing movie of 2012 so far is The Avengers, the second top grossing movie of 2012 is The Dark Knight Rises. The fourth is The Amazing Spiderman. All of these movies have basically the same plot...superheroes battle supervilliains to save the world. Yet the people who saw Batman didn't say, "Well I saw Batman so I won't go see the Avengers because I've already seen a superhero movie this year." On the contrary, they flocked to see all the superhero movies. And they knew what to expect in those movies even before the movies were released. They knew that the same elements of story would be repeated in each movie but they still paid for a ticket because those elements are the reason they like superhero movies.
Tropes are devices built into your genre to attract readers. Give those readers what they want and they will come back for more.
There is an excellent and very comprehensive site on TV tropes here:
Scroll down the page to the 'Genre' bar on the left hand side and have fun!