Whenever you build a secondary world, a fair amount of your book is going to necessarily be devoted to world building. This is true regardless of whether or not you are doing a low, portal, or high fantasy. After all, you have to introduce your world to the reader. And the more different the world, the harder it may be for the reader to visualize. The easiest way to describe something is to relate it to something you expect the reader to know. This is why figurative language can be so effective. But if you have a really different world, it can be hard to describe it to others especially if you world requires new slang and jargon.
So, most high fantasies are written in third person. Even with a close character point of view, there's a little more room to fudge when you're somewhat omniscient. You have to be careful, but by using figurative language and other cheats, you can relate your world back to ours. For example, if you have genetically engineered animals in your world, but the world doesn't call it genetic engineering, you might be able to mention the term genetic engineering at some point in the narrative. It would depend on the tone and how jarring the term is to the overall flow of the text, but at least there is a chance you just might be able to sneak it in.
This is not an option in a first person narrative (or for that matter a third person point of view that is so close that it might as well be a first person like the Harry Potter books or Lyra's portion of The Golden Compass). In these books everything has to be consistent with the character's knowledge. Using the example above, if the character has never heard the term "genetic engineering," then the term had better not ever pop up in the text. The character simply wouldn't have thought of those words. If you use them, you've broken character.
What's worse for a first person narrative (and not for a third person regardless of closeness) is that most first person narratives assume that the reader is also a member of that world. So, there is no way for there to be references from our world because not only would the character not realistically think of them, the reader shouldn't be able to understand them.
So, if a first person high fantasy is so difficult to write why does anyone do it?
The answer is that most don't. Most high fantasies are in the third person because ultimately in is a more natural way for the author to write. You have more opportunity to relate to our world and an implied reader who is of our world, not the secondary world.
However, there are a few books that take on this challenge, and they do it well:
- Homeward Bounders by Diana Wynne Jones
Jones gets around the whole reader being a part of the secondary world by having many secondary worlds in this book. The first person narrator jumps from world to world in his quest to go home. Since he's supposed to be recording a record for someone who didn't know that the other worlds existed, Jones is able to explain all of the worlds. And although the narrator doesn't come from our world (hence it is a high fantasy -- technically the book is a science fantasy) it is close enough to our own that we understand what he talks about.
- Sorcery and Cecelia or The Enchanted Chocolate Pot by Patricia Wrede & Caroline Stevermer
This book is set in a world that is similar to Napoleonic Britain -- but a world with magic. The two characters are writing letters to one another, and so they assume the reader (the other character) is already acquainted with the world. However, the world is not so dramatically different that the reader has no difficulty catching on, especially if the reader has read any Jane Austen.
- Flora Segunda by Ysabeau Wilce
This world is not even remotely similar to ours, and has very few, very slight parallels to our own. In fact, at some point I decided that the setting (the Republic of Califa) is our California. Is there any reason for me to think this? No. It's just my brain's way of trying to relate the world to our own. That or the clues are so subtle that I picked up on them subconsciously. Either way, the book's world is fascinating, and the main character jumps in with full slang from the first sentence. There's no doubt that this world ain't our Earth anymore.