Now, I'm not saying "YOU CANNOT PUT THESE IN YOUR BOOKS!". I'm just pointing out some commoners.

1. Your protagonist is white. I have seldom seen any exceptions to this chlech'e. It's sad. And let's not forget that if your main character is Asian, he or she MUST be able to do martial arts. I know a few Asian people, and so far, none of them do MA.
2. Your heroine/ hero's girlfriend MUST be impossibly skinny. If you've ever seen Meet the Robinsons you'll get the idea. It's a miracle that Frannie actually was able to deliver Wilbur. And she MUST be a fashion model.
It is very unrealistic.
3. Same goes for heroes. Incredibly handsome.
That's one reason why I like the original Aragorn.
4. When the hero goes in to fight, please be aware that there are two types of fighting.
A. calculated. Find your opponent's weak points and use them.
B. RAWRRRR! *makes like the hulk. or Wolverine*
C. Pleasepleasepleasepleasepleasedon'tletmedie! (yes, that IS a fight style. Ask some one who's been in a real fight.
5. wise, old mentor dies in front of the hero.
6. hero's girl friend is in grave danger, and has a front row seat to the 'epic' battle between hero and villain, while doing nothing for no reason.
7. little kids can solve mysteries that adults can't.
8. LOVE TRIANGLES!!!! such as the matched trilogy. That went on forever. except that by the end, it became sort of a love hexagon. or something like that.
9. speaking of love:
Heroine: "oh, Will, I love you sooooo much! Did you notice that I committed treason, ran away from daddy, nearly killed a man, and let go of my position for you?"
Hero: "I love you too, Elizabeth! You're so nice!"
4 days later...
Heroine: "Oh, Will, I used to love you, and you're still a nice guy, but you're so...tame, compared to Jack over there."
Hero: "What?! Okay, that's it! We're done!"
and in the end they get married, and he leaves her on a dessert island.
10. Farm boy who is unknowingly a prince. That's what made The False Prince so cool.