- No Big Bad Evil guy who wants/is able to destroy the world. Politics provide enough conflict while leaving open the option of failure without everything going to hell.
- No build in "awesome but silly moves". Keep your buster swords your whirlwind attacks and nonsense acrobatics, thank you very much.
- Appropriate to its time period. While there is certainly some leeway, especially when magic is involved, in a world with medieval technology not every town should have a sewer or a library etc.
- No "Humans with pointy ears = good, rest evil". You can have monsters, yes. But everything capable of reasoning should not be reduced to good and evil, especially along Tolkien lines of alignment.
- Less Tolkien in general. Its simply overused.
- No "adventurers are special" treatment. If adventurers can use it so can everyone else which should affect the world in a appropriate way depending on availability (can be read as the often mentioned magic as technology").
But as I am nearly always in the minority, if you want to earn money do the exact opposite of what I want.
Edit: And consistent naming. The names of locations and persons should be consistent with the culture that named them instead of a random string of letters.