Let's say you grew up on Dungeons & Dragons (or other similar role-playing games like MERP). You love that type of world and you now want to write about it. You people your world with all the elves, dwarves, goblins, trolls, and dragons that you loved so much from your gaming days. You know in advance that a lot of readers will be turned off by this, since they've seen it a few times before and these readers never like to read anything similar to what they've read previously, but you aren't worried about that because you know there are still lots of readers out there who do like these types of stories.
So, you write and you write, and eventually you finish. Then you are dismayed when all your beta readers keep telling you how so many scenes remind them of Tolkien. How can this be, you say, since you were not thinking about Lord of the Rings when you wrote your story.
Well, it's simple really. Tolkien did it all (or pretty close to it). His books were so in-depth that as far as taking a party of mixed races and having them move about on a map (in pursuit of a grand, epic adventure, naturally), you are pretty much out of luck if you don't want anyone to draw any parallels to Tolkien. Want to move from one place to another? Okay, have the characters walk. Oops, Tolkien did that. Ride horses or ponies? Check. Ride in a cart? Check.
How about you come to a river? Take a ferry across? Nope, he's done that. Find a ford? Been there, done that. Ride a boat down the river? Cross on a bridge? Check and check.
Swamps or marshes? He's done it. Forests? He's done it all. Mountains? Cross in a pass? Find an unknown path? Find caves that lead you through it? He's covered them all. You might as well just have some big eagles swoop down and pick you up, but he's done that, too. I guess you could have the characters rig up a hot air balloon to carry them across, but that, at least to me, is a more modern version of fantasy than suits my D&D-inspired tastes.
I got some spot-on feedback on my book recently, and I am grateful as heck for it. I will really be able to examine some scenes and figure out a way to make them a little more unique. The only way it bothered me was in knowing that I had already tried to avoid Tolkien's more famous stories, yet some scenes still reminded the reader of parts of The Hobbit or LOTR. The fact is, much of my story was inspired not by either of those books but rather by the part of Beren's story in The Silmarillion where he escapes from Dorthonian by crossing under the mountains. That piece of his story always stuck with me, because Tolkien teased us by saying that Beren endured much horror during the crossing, but he didn't go into specifics. I always wanted to do a story that dealt with an extremely hazardous crossing beneath a mountain range. I didn't intend it to be like Moria.
The problem is that there are only so many realistic variations of passages under a mountain, and Tolkien pretty much hit them all. Rough stone, worked stone, small rooms, big rooms with pillars for support, chasms, bridges...there's not much else you can realistically work with, yet Tolkien did all those.
My reader was right, though. I may not be able to avoid Tolkien altogether, but I can at least ensure that big moments of the journey don't fall within similar settings. In other words, if I currently have an important battle taking place on an underground bridge, it's probably best to move it to someplace else so it doesn't make readers think of the ending of the Fellowship movie or book. It would have been embarrassing to me to have the book published and only then to hear from readers about such similarities, since I didn't know they were there, so to this friend I say, "Thank you."
To those who would say, "Well, don't use elves and dwarves and such.", I can only respond that this is my passion. I spent years playing D&D and living within this type of world. I always rued the fact that no one was ever publishing any stories set in such a world that dealt with it in a truly serious manner. Tolkien took his world very seriously, and that's why so many love him. The official D&D or Dragonlance novels? Not serious (at least in my view).