2) Make your magic clear and consistent: This is actually the cardinal rule for any fantasy: if your reader doesn't understand how magic works in your world, or if you change how it works midway through—using some kind of magus ex machina you pull out of your hat to rescue your characters at the last minute—your readers will feel cheated, and you will undermine your big magical payoff. Or, as in the case of Unnamed Book, your reader won't realize there was a big magical payoff. While one kind of magic was portrayed very clearly—dreamwalking—it was a relatively minor magic (in terms of power); when it came to the major magics—the earth-swallowing, impenetrable barrier-creating kind—I wasn't sure how it was done, who could do it, or how you could battle it. So when the heroes finally had their big battle against a major magic, I wasn't clear how they were battling it and what the effects were. I actually thought one of the characters had been left in a coma by the battle, when it turned out she'd only gone in a different direction from the hero. So I was left feeling confused, unmoved, and unimpressed.
On the other hand, consider Ursula K. LeGuin's classic Earthsea series. Her magic is simple yet powerful: learn the true names of things and you can learn to control them. It's a simple, understandable concept, yet she uses it build various magical confrontations that are clear to the reader. Because we understand how the magic works, these confrontations have more tension and more impact. Your magic doesn't have to be original—think of the Percy Jackson & the Olympians or Artemis Fowl series, which make great use of Greek mythology and Celtic fairy legends, respectively—but the reader should be able to understand it.
3) Be aware of character archetypes: this can be a really big pitfall, especially if you aren't straying far from the familiar in terms of plot. It can crop up in the work of the most devoted fantasy reader/writer, too; I remember reading the first volume of one bestselling series, written by an author who was a fantasy fan, and about two thirds of the way through saying, "This is Star Wars, but with dragons!" The further I read, the more character archetypes I could identify: farmboy with a talent (Luke), spunky princess (Leia), secretive mentor (Obi Wan), mercenary with a heart of gold (Han Solo), oppressive ruler with some strange connection to the farmboy (Darth Vader) ... I kept reading and kept drawing more parallels, because the plot was actually very close, and it got very distracting.*
Now, that wasn't the case for me with Unnamed Book. I thought aspects of the plot were original enough that it didn't remind me of anything else in particular. (Maybe I haven't read widely enough, for one reader review on Amazon did complain of plot similarities to another popular fantasy novel.) But the traditional fantasy archetypes were all there: the Fatherless Young Man trying to prove himself; the spunky Priestess/princess; the thief/mercenary with a heart of gold; the mentor keeping a secret. Of course the first two characters had a sparky love/hate thing going on throughout the book, so of course I knew they would end up together at the end. That's not a bad thing (that's why we love romantic comedies), but I guess I felt dissatisfied because I didn't feel like the characters grew beyond the archetypes. For example, a big part of the ongoing love-hate thing was because Princess tricked FYM into rescuing someone from a prison camp, and he had lingering resentment from her "betrayal." But if Princess knew anything about FYM, it would be that he had severe daddy issues, and all she needed to do was tell him that the someone in need of rescuing was his long-missing father, and he would have volunteered to go in without any trickery. Either ignore the archetype, or use it and grow beyond it, but don't use it and then ignore the major character attributes attached to it.
For a great way to rise above archetypes, just look at Lloyd Alexander's five-volume "Chronicles of Prydain," which I think I read 20 times by the time I graduated from high school. The series' main character Taran, the hero who goes on several quests, is introduced to us as an orphaned assistant pig-keeper. He encapsulates the FYM archetype perfectly: worried about his identity, striving to prove himself, overconfident and yet unsure, overly impressed by power and rank. Over the course of the series, however, Taran learns different values and grows into a young man who values ability over nobility and comes to see war not as a chance to prove himself, but something to be avoided. I can read his story for the umpteenth time, even as an adult, and it never fails to affect me.
Now you might point out that Some Author of Unnamed Book has only written the one fantasy, while many of the "right way" examples I used are fantasy series. Rowling had seven volumes to detail her world, and Alexander five to develop his archetype. So am I being unfair? I don't think so; after all, if you can't manage to make the first volume work, the less chance you have of getting the next ones published.
Rant is over. Now if you'll excuse me, I feel a nervous urge to go revising.
*This is not to say your fantasy plot has to be the most original thing in the world. I attended a talk by Philip Pullman, author of my favorite fantasy His Dark Materials, and he claimed there are only 10 basic plots in fiction, and all of them are variations of the "Holy Grail" quest. But still, follow the most famous space fantasy film of all time event by event, and people will notice.