Let's take a look at the world part of the story first. It could be the world we know and are familiar with, only with magic behind the scenes (or not), or it could be something completely new, created from scratch by the author. One of my favorite examples of this is, that I've read recently anyway, (of course) the Shadowmarch series by Tad Williams. (Forgive me talking about him so much in these posts, I just really like his books. *shrugs*). He started from scratch and made up, not only a kingdom, but a whole world filled with gods and demigods, human characters, and various forms of magical/mythical beings. Writers who create new worlds go through quite a bit of work to make it happen. I for one love to do it. I have created new worlds multiple times in my WIPs. It can be daunting and sometimes hard to figure out all the details needed, but in the end it's always worth it. To me anyway... When a writer creates a new world they have the opportunity to go beyond the boundaries set for them by the world we live in. There are limits, of sorts, that come with writing about our world. But when you create a new one, there are no restraints. Granted, it has to be somewhat plausible, but there is far more room to move when you can create everything down to the tiniest detail; make it all your own design.
Ah, now here's where I really get long-winded and rambly. The plots, the plots, the plots. Love 'em. Not always mind you, there are some books I've read that I wished I'd never even heard of, but for the most part I do love them. Of course the plot is essential to any story, that's just a given. But what I'm referring to is the fact that pretty much every story has been told by now, it's the ways they are reinvented that really excite me. Who hasn't read various "chosen one" scenarios or the whole apocalypse idea over and over again? But why do we keep reading these stories that, at the core, are all the same? We do it because the author has the ability to make their story different from all the other ones out there. We do it because the author is able to take an old idea and put a new twist on it. Let's talk Harry Potter for a moment. Another "chosen one" situation, but done different than the rest by the tiniest details that make up the characters, the setting, the trials, etc. Inevitably one side is going to win in these battles, and it's not always the good side either. In my reading experiences, which admittedly is much less than many people I associate with, I have found that I like these chosen one stories that deal with the younger generation. Depending on how long these things go on for, years in the case of HP, we may get to see the character grow and develop, either into who everyone thinks they should, or into the polar opposite of that. After all, the chosen one doesn't always turn out good... or even begin that way. Watching how the character deals with the burden on their shoulders is also another appealing thing to me. It makes me wonder how I would cope if I were ever in the same situation. Though, I will say, it wouldn't be nearly as well as they do. :-D
How many times have we read a book where the fate of the world rested on the MC's, and possibly supporting character's, shoulders? I have done this a few times myself. The appeal for the end of the world scenario, to me anyway, is to see how the MC overcomes it all to win in the end....or not. There's a 50-50 chance that things will not go the way they should, i.e. HEA or at least the best HEA possible.
There's also a chance the world will crumble and fall, taking every inhabitant along with it. Don't ya just love it? There is also to consider, if the apocalypse is averted, the aftermath of such a battle. What does it do to our everyday lives? What does it do to those who, thinking magic is nothing but fairy tale and legend, see it firsthand and must live with the knowledge of something they've never faced before? Not only that, but let's say the MC survives the whole ordeal, how do they go on afterward? What do they do now that, in some cases, their destiny or purpose is complete?
Now, I know you don't always get the answers to every question you have when you read a book. Sometimes you're left scratching your head and wanting to give anything just to get more information. But that's the beauty of it. If a story can leave you wanting more it's more than worth it in the end. You may be frustrated with the way the story ends, you may not agree with what happens, but that's just the way it goes. If with your story you can manage to bring your reader into your world, make them feel like they are living through the same experiences as your MC, and sometimes even make them want to be your MC, then IMHO you've written an excellent Fantasy story. Even if your preferred genre is not Fantasy, I would recommend giving it a shot at least once. You never know what's going to come of it.