As long promised and taking a long time to deliver: D&D stereotypes!
Once again, as with Tolkien, it’s not really the fault of the people who created the games (though it might be the fault of the people who wrote novels that tie in with the games and adopt their rules unthinkingly). It comes of a lot of authors playing D&D and, essentially, thinking that’s how you write a fantasy novel.
Pssst. It’s not.

1) Default= human.

One of the original parts of the D&D set-up (it may have changed now) was that humans got to have lots of special advantages and powers as player characters, to offset things like elves having longer lifespans, dwarves being physically tougher, and so on. The unfortunate side-effect of this is that human gets treated as the default, and “demihumans,” the name for races like elves and dwarves, are seen as variations on the human, instead of unique races unto themselves.
The effect of this is everywhere, and it’s not unlike the science fiction cliché of just happening to have one team member of every possible ethnicity. There has to be an elf, who will probably sing and be in touch with the trees and be a good archer. There has to be a dwarf, who will like ale and be gruff but gold-hearted and swing an axe. (Dwarves appear to have no other purpose in most D&D settings). There will probably be a halfling, who will be, of course, small and sneaky. There might be a gnome, who’s an illusionist because…just because. All of them will have less of a personality than the humans, who get to be not only [insert profession here], but neurotics and cowards and heroes and villains, too.
This makes sense in a game setting, where people want the options to play whoever they like, and the GM wants the ability to somewhat equalize things between the players. It does not make sense in a fantasy book where the author is using the cliché without thinking, or in an attempt to say, “Look how multicultural I am!” And no, you can’t point to Tolkien on this one either. His party didn’t attempt a “balance” of races in the way that a lot of books employing this stereotype do; otherwise there would have been as many Elves and Dwarves as Hobbits, and more Men than either, and an Ent would have come along too. Tolkien’s characters went along to represent their races’ concern in the battle for Middle-earth. Most fantasy characters are in parties because their author thinks there has to be an elf. There was in their last campaign, after all.

2) Hordes of generic bad guys.

All right, perhaps you can somewhat point to Tolkien on this one, since he did create the Orcs and turn them loose. But D&D has exacerbated the problem. There are always, always more Orcs. While some attempt is made to document human population levels, and even the levels of other races—like the idea that the dwarves are declining in a lot of D&D worlds—and to explain why humans or elves or whatever serve evil if they do, Orcs are just There. And they are Evil. That is the Way It Is.
This also leads to the idea, I think, that heroes can kill the bad guy’s servants without guilt. They know in a campaign that they’re expected to agonize over killing someone who’s only fighting in self-defense, but an orc, a goblin, a hobgoblin, a bugbear, or a human who’s proven to be scum working for the enemy? Take ‘em down!
This leads back to the superficiality of death, which I’ve touched on before. Death really only matters in a fantasy book like this if someone on the good side dies, the way that most players will mourn their own player characters but don’t give a fig about the people they go around spitting. If a hero is scarred from war, it’s always because of the death of comrades or because he was fighting against other humans or one of the demihuman races. Go against evil guys, and he’s fine.
Consider what happens to soldiers in our world, even soldiers fighting against enemies they’ve been told are inhuman. There should be problems with death, confrontations with war, and it cheats to insist that only some races are worthy of mourning, while others are inherently evil and not to be bothered with.

3) The goal is power.

For a lot of RPG’s, it is. The character gains experience points and with them, levels. Thus they “learn” how to do new things, perform new spells, gain greater abilities, and, not incidentally, amass more and more money and power and influence in their societies. And for a game where the focus is on the development of the player characters, that’s great.
Again, though, game-playing is not writing fiction. I’m sure you’ve read at least one fantasy novel where it seemed all the protagonist did was gain new and cool abilities, sometimes without having to work for them, and at the end saved the day with no trouble, since now he was as strong as the Dark Lord. Then he went on to rule wisely and benevolently, and while it had to do with power, of course, everyone respected him because of the power. No one feared him at all. Oh no, of course not.
Trouble is, with well-written fantasy you’ve got a whole other world out there. The characters, written well, cannot be trusted to act like NPC’s in a game, who might hinder the player characters for a little while, but are rarely allowed to hurt or kill or permanently trample on them. They’ll have their own agendas. The hero can manage to outthink, outwit, outfight, or outgun them, but the author should not take a game-playing ethos and infer that, since the goal is to make the character as powerful as possible, he’ll swat everyone out of the way with only minor trouble. If no one can touch him, he’s at least a transplanted demigod-in-the-making, and quite possibly a Canon Mary Sue.
Fantasy has enough of a problem with that already, with some people uninfluenced by D&D deciding that “Magic doesn’t have to have rules!” and giving their heroes the ability to do literally anything. Avoid this extra trap. Please.

4) Mage trappings.

The ideas that power comes with seniority (levels again), that mages read spells from books and then forget them as soon as they cast them (a means of limiting the power of mage player characters), and that most spells need chanted words and components are all strong in D&D. And they get into fantasy and influence them, and then you have tried and tired clichés like the ones I ranted about in the mage hero post. Even worse, some authors think that these ideas don’t need to be explained because they occur in games, and just have their mages follow the system, whatever the holes in plot logic that produces.
Consider this: What’s going to happen when the mage runs out of components, or loses some? What happens if someone steals or burns the spellbook? What happens, as people suggested in comments to the last post, if the party really needs two fireballs but only has one mage, and the mage, having cast the spell, forgets it?
Take the rules away from the world where they’re supposed to work—a world with arbitrary, imposed limits meant to serve gameplay—and they really don’t work. If you have a magic system that uses chanted spells and components and spellbooks, really think about why, instead of using elements or ley lines or magic of the will or telepathy or any of the thousands of other systems out there. If older mages are the most powerful, consider why. Does being alive longer really make them wiser, and even if it does, does that mean that they would have more ability to wield magic? Knowledge does not necessarily equal power, any more than it does in the real world; politicians can get by with being stupid, while academics can rot in their ivory towers. If you use a magic system with different levels of talent, a hot-headed young mage might actually be more powerful, if uncontrolled, than a man who’s studied all his life just to harness what meager gift he has.

5) Tons of wandering adventurers.

D&D heroes often have tragic stories that drove them into the lives of wanderers, such as watching their entire family be destroyed in an attack on their village and then taking up the quest for vengeance against the evil person who sent them. (You’d think the Dark Lords would really learn to kill everybody in the village, and not leave the conveniently vengeful teenage girl or boy still alive). Then they travel about, amassing wealth and fame and legends and songs about them, and thus they’re free to randomly save the world when the game needs them to. No family ties bind them. No inconvenient people are popping out of the past to hold them back, except perhaps old enemies that the GM wants to use in the current storyline. Common people bow in awe of them, and they’re accepted as part of an almost automatic class of heroes, despite the damage they leave behind them.
Really, does this make any kind of sense in a more carefully-realized fantasy economy, one where the purpose is fiction and not gameplay? Not really. There could still be classes of wanderers, but they’re unlikely to find much employment as heroes. They could be mercenaries, thugs, highwaymen, pirates, freelance killers, thieves for a living—as opposed to the stylish thieves who seemingly steal mostly to see if they can get away with it—or any other occupation that would let them keep bone and flesh together. But it would be the very rare person who didn’t have an ordinary supply of money and yet managed to live anyway, or whose reputation was so widespread as to garner him or her free drinks and food everywhere. Consider a medieval fantasy world, a carefully-researched one. Travel would be rare and expensive, and wandering minstrels would sing about people who did really great deeds, not just saved one village girl. Even if someone who was homeless and wandering did a really great deed, it would be more likely to get taken up and attributed to a knight or a royal, the same way that many deeds in legend tend to cluster around a single group of people, like Arthur and his knights. Let the adventurer just try to get a free drink in a tavern a few miles from home. It’s quite possible that no one would know who he is.
Also, consider what settled villages or townsmen are going to think when this wanderer comes among them. Has he dressed in fresh clothes recently? Probably not. Does he carry a sword? Oh yes. Does he leave a trail of broken bodies behind him? Most adventurer heroes do. I don’t think a lot of town councils, mayors, kings, governors, or what have you are going to like that, to say nothing of the common people who have to deal with this ruffian.
Point boiling down to: In games, the adventurers can work because the campaign is focused on them, not on the world as a whole, and “adventurer” is an accepted occupation. In most fantasy worlds, it’s not, and the author is supposed to be designing a whole society out there, not only the part that spins around the characters.
I’m sure there are more of them, but this hits a lot of the important ones, I think.