Of course I can’t possibly think of every single inconsistency that I’ve seen, but these are the biggest. And most of them are easily preventable. (I sound like someone trying to urge vaccinations on people, don’t I? “Do this, and your novel will be safe against the pox!”)

1) Hello, wounds. Stick around for a while, and then go away when I tell you.

A lot of fantasy authors forget that someone is wounded or sick. In some ways, this is understandable, since we tend to deal with pain by taking medicines, traveling to the hospital, and doing other things not available to people in a medieval fantasy world. On the other hand, wounding your hero and then curing him without explanation smacks of wanting to give him the “sympathy card” without accepting the consequences. If he gets a piece of wood stuck through his foot and there’s no mage available to heal the wound, don’t forget to have him limp. If he has heat exhaustion, he shouldn’t just jump up the next day and run to catch those evil horse thieves. If it’s a big plot point that your heroine suffers menstrual cramps every single time she bleeds, then she shouldn’t suffer them one month and the next month walk around smiling merrily at everybody. Adjust your mental image of your character from “whole” to “suffering” and write him or her that way for as long as it takes to heal the problem.
Speaking from personal experience, one of the hardest things to do when you maim a character is to remember not to pluralize words you’re accustomed to pluralizing. If the character’s missing a hand, remember not to have him dragging firewood around “in both hands.” If ravens have plucked out his left eye, remember that he can’t glare at anyone “with burning eyes” (oh, and that he has a blind side). There might be very good plot reasons that you did this, but if you don’t treat them with respect, no one else will be compelled to give them much respect either.

2) Know at least some basic ecology.

This might be entirely nitpicky on my part. Well, so be it; condemn me to nitpickiness. I know that it drives me nuts, so it’s getting mentioned.
hate it, with big fucking cherries on top, when the author has tigers wandering around in the middle of temperate woodlands, or trees growing tall and strong on windswept plains, or characters walking through “deserts” where freaking rivers are running. No, no, no. You take the consequences along with the good stuff. If you want your readers to admire vast vistas of windswept dunes, then you also have to write admirably about a lack of water and your characters needing to conserve it. If you want tigers, you have to change the setting of your story from pseudo-England to pseudo-Africa, or pseudo-India or pseudo-Siberia if you have specific subspecies of tigers in mind. If you want lots and lots and lots of sunshine, you can’t also have lots and lots and lots of rain—or at least not on the same days the sunshine happens.
This is the most obvious thing you ever heard. Well, it’s not the most obvious thing a lot of amateur fantasy authors have heard, I’m sorry to say. Here’s a tip: Before you put a specific species or subspecies of animal or tree in your world, go and check that it could actually survive in that kind of environment. Before you have trees growing tall and strong, ask where they’d get the nutrients, and if the wind wouldn’t stunt them. Before you have rivers flowing every which way, ask if they’re flowing uphill in defiance of gravity, which still applies in most fantasy worlds. Fantasy authors have to ask not only “What if?” but “Why?” constantly.

3) Your readers can’t read your characters’ minds.

They know only what you show and tell them. That you, as the author, know vastly more about these people than anyone else cannot be allowed to control the story. It’s why it’s you writing the story, but if you don’t make it clear why these people behave as they do, it will also be why no one is reading.
By all means, keep your characters’ motives obscure for a while if you must. But at some point, they need to come clear. And by the leaps of imaginary tigers in England, don’t state firmly in one part of your story that the princess is chasing the hero because he killed her father and she wants revenge and that is her only motive, and as firmly later in the story that she is in love with the hero and has been since childhood and that is her only motive. She might think that way, but you have to offer the reader a way to reconcile these two disparate things. You can hint around at the beginning that the princess is going to fall in love with the hero, you can have childhood memories in there, you can have the princess feel confused about her revenge even as she pursues it. (This is why it’s good to develop a gift for showing that your character’s perceptions are not identical with the world about her). But you can’t have her go effortlessly from revenge to love, or vice versa, dropping one motive like a hot potato when it’s no longer convenient.
Personally, I think a lot of fantasy authors would do better to forget about the mystery that they fill characters with for no given reason. They’re not good at it. And too often, when they get caught up in writing the fantasy part of the story, which they are good at, they forget that the characterization they just gave contradicts what they said earlier in the story. The result is characters with seeming MPD, and very confused readers.

4) Events that happened before the book are important.

Keep your stories straight about these things, please, and other events of like importance. I’ve read too many fantasies where the heroine believed on page 3 that her father had gone wandering off to follow the call of adventure, and believed on page 20 that her father had abandoned her mother because he was a bastard.
Again, if you don’t keep your stories straight, you have to offer the reader an out. Stories told by different characters are the best solution here. The daughter might believe one thing, the mother another. But even there, you have to explore the differing stories, and compare them, and try to offer some explanation about why those two people, who have certainly spoken about it, believe such disparate things. And if one character believes two contradictory things, with no comment about why she does so…yeah, one plausible explanation needed over here, please. And it better be damn plausible.
This is the reason that you should decide on one explanation, and keep it there. You should know the truth, even if not all of your characters do. That will give you license to play around, decide who would have a chance of knowing and who wouldn’t, what kind of stories would spin out from it, and why certain people might prefer one tale over the other. But too often, I get the feeling that the author really doesn’t know, either, or is trying to avoid making up her mind until the last possible minute. At some point, though, you get enslaved by your choices; every important decision you make about things like the structure of the story or the character of your heroes and heroines shuts off other possibilities. Storytelling springs from that happy bondage as much as it does from the initial freedom. Stop worrying about coming up with a “cool” explanation and settle on a good one, and if you can’t, then abandon the mystery.

5) We’re already lost in a welter of unfamiliar words. We don’t need to memorize unnecessary ones.

This is an especially big problem in the looming epic fantasies, but even in smaller fantasy novels with notably alien settings it piles up. This is the practice of introducing, say, an order of mages called the Guardians of the Night and then insisting that they are also the Black Brotherhood, the Ebony Cravens, the Wings of the Stormcloud—and then using all those names to refer to them in different parts of the book. Or a nation is not only Seurenna, but also the Blasted Land, the Cursed Country, the Barren Desolation.
Or, my least favorite example, a character is not only “Shilanna,” but also “the Prince’s daughter,” “the green-eyed woman,” “the Prophesied Princess,” “the vibrant young lady with blonde hair.” I’ve read many books where it took me a hundred pages to realize that about twenty-five different descriptions actually applied to only four characters.
For the first two examples, you shouldn’t depend only on glossaries, and you shouldn’t try to insist that your readers know all these names. Spread them throughout the book, or assign them only to certain people. Perhaps the Guardians of the Night are referred to that way by their friends, and their enemies call them the Ebony Cravens. Perhaps the Blasted Lands is an exaggeration, used by villagers who fear that land, and Seurenna is its more learned name. At the very least, don’t expect us to memorize all these names at once.
As for characters…
I am a great fan of proper names and pronouns. A very great fan. I hate all this “green-eyed lad,” “blonde-haired lady,” “the Princess with the large melons” nonsense. I read enough of that crap in Harry Potter fanfiction. I don’t need it polluting my fantasy as well.
Read the following paragraphs, and tell me which of the two is less confusing:

Kran sat in the garden with Shilanna and closed his eyes, the better to feel the sun on his face. Shilanna shifted beside him, and he reached over and clasped her hand, a slight smile forming on his lips. He knew that one on her face would answer it, but he didn’t look. He just leaned his head on her shoulder and sighed. He would have to leave soon enough, and though she had tried and tried, she hadn’t been able to persuade her father to rescind his sentence of exile.

Kran sat in the garden with Shilanna and closed his eyes, the better to feel the sun on his face. The Princess shifted beside him, and the ebony-haired youth reached over and clasped the younger woman’s hand, a slight smile forming on his perfectly curled lips. The Princess’s suitor knew that one on his beloved lady’s face would answer it, but the broken-hearted lad didn’t look. He just leaned his head on the fiery-tempered young woman’s shoulder and sighed. The green-eyed son of the Drakefire line would have to leave soon enough, and though the Princess of Stones had tried and tried, she hadn’t been able to persuade the King of Seurenna to rescind the royal sentence of exile.

Yes, that second one is a slight exaggeration, but not by much. And I am going to think, at least on a first reading, that some of these descriptions apply to different people.
Finally, I think that most readers will get along without you reminding them of the characters’ personalities and professions and eye and hair colors every twenty words. There’s being respectful of your readers, and there’s assuming they have the attention spans of mosquitoes.
Basically, be careful. And yes, that’s easy to say and hard to do, but fantasy is worth the effort.