1. Over-competent characters. You get to be the greatest strategic mind that ever lived or you get to be a genius-level mathematician. Pick one. You are either the abovementioned strategic genius or a supremely gifted musician and the best lover in the three nearest galaxies. You are allowed to be the unbeatable hero and a passably good lay especially for your sense of humor, but you have then to be at least crap at playing the piano, and/or routinely burn your boiled eggs.
2. Twentieth centuries trappings in the far future. The diner. The Starbucks, yay, verily, even if it called Mamoushy's. Characters getting up and having breakfast with bacon and eggs. Eating spaghetti. And/or apple pie. Coffee is allowed, because, obviously, it is necessary for the continuation of the human consciousness. But not tea. Especially not Early Grey. OK, tea is allowed, but pains must be taken to specify where it is grown and its long-established tradition. No cheating with the Andromeda Darjeeling in this case. Oh, and this goes for Specific American Concerns in the Far Far Future as well. The far future is a different country, and different countries are politely uninterested in many, many things Americans in the twentieth century find riveting. So if you can find a larger, universal meaning to it, go ahead! But if you want to write about gnu control, underage drinking, or whether a public health system is really such a good idea, then be aware that most of the rest of the world has generally settled these things to their own satisfaction long ago. Don't get me wrong, I am utterly riveted by the American political scene, and I read a lot about it. But I don't think its specific concerns will last longer than the present century, if that.
3. Swords in the future, and not even the far future. There is a reason the sword is not in common usage nowadays. It was never a very good weapon, it was just a reasonable compromise, and as far as I am concerned, projectile weapons rule. Complicated scenarios conceived just to bring back the sword are no go either. Exceptions can be made for katanas, but very, very reluctantly.
4. Using cultures you are not absolutely totally foolproof familiar with because they sound cool. The next book set in a Renaissance Italy with names incorporating non-Italian phonems like that get unstitched, covered in tar, feathered, and burned at the stake. This seems to be in contradiction with point 2? It's not. There's a whole world out there. Explore it. Reinvent it. Steal form it. Don't steal it.
5. Using culture-specific names and traditions in a non-mimetic world. I can forgive George Martin for having characters named Robert and Jeoffrey in a place that has no reason to have such names, but not anybody else.
6. Female character whose main motive is love. This is because I have read, in a collection of a definitely great and classic author, a truly really horrible stinker of a story about the Heroic Love of the Tragic Heroine. You know, the woman who is the first to sail a ship... because she wants to meet her True Love. Love is ok. Hell, lots of Tiptree character act out of love... although "acting" in this case seems mostly to be synonimous with "kill themselves and sometimes their whole culture and/or race". But even Beren and Luthien had other motivations, as well.
7. Gratuitous cat scenes. Ok, I will amend that. Gratuitous cat scenes are ok, in fact I have done many and I will keep on doing it. But the cat has to be a real cat, and not be telepathic, funny colors, or an alien animal that just about happens to look and behave like a terrestrial cat. Same goes for horses. No animal bonding in general.
8. Plots about how fun it really is to be dominated. Sorry people, and spoken as a bottom, it's not. People actually get off from real torture, but none of them are victims. (I have, in fact, written a story about somebody getting off being raped, but a) that was porn and b) the point was a tad more complicated than that). Come to think of it, characters who angst endlessly about how cruel they have to be, not that I am looking at you, Susan Matthews. Be a respectable villain and enjoy your torturing, or get out of the game, yeah, verily, if it costs you your father's respect or a bullet in the head. And as an aside, and this is IMPORTANT - it's not bad because you enjoy it; it would be bad just as well if you hated every moment of it. The problem is not that you like it, the problem is that you do it.
9. Since we're at it, terrible torture that consists of being slapped in the face, or equivalent. Do your research people. Virtuality is no excuse - Richard Morgan could pull off truly frightening torture even in cyberspace.
10. People who keep slaves but are really really nice. You should especially not do it if you're a white American. Strangely enough, I know of very few African Americans doing this kind of stuff. Funny that. Do we need to mention the all-white cast, the Token Foreign Character with the Funny Accent, the Wily Chinese, Precise and Cold German, or the Honourable Japanese here? I thought not. Ok well, just because, I will mention the Corrupt Superstitious Volatile and Irrational Italian.
11. How jolly fun killing beings is OR how utterly dreadful war is. I sympathize more with the second position than the first (although I have blasted away my share of virtual beings with great glee), but I think ignoring the other side of the coin is too easy and too glib, although the first is by far more morally inexcusable. As Auden said, "But for him I could forget the blood, but for me he could forget the innocence".