1. No Fucking Elves.
This is my big number one. Now, what do I mean by that? I don't mean it literally, that any story with an elf will be no good. But odds are high.
Why is that? Because, in my experience, it's usually a cheat. People will have "elves" as a shortcut to avoid doing real worldbuilding and society creation work. Too many works just say "elf", with the idea that it's understood that said elves are forest-dwelling, bow-wielding, tall-and-graceful, long-lived ponces. Likewise with dwarves, trolls, gnomes, orcs-- and to a lesser degree, werewolves and vampires. Using this stuff just makes it look like you're cribbing off of Tolkien. Or worse, cribbing off of D&D.
And just renaming them something else... that doesn't work for me either. I've had some writers tell me they have their X-race in the Forests and Y-race in the Mountains, and then in a low whisper say, "You know, my elves and dwarves", like they KNOW they're cheating, but they hope they are getting away with it.
Now, as with all things, it's not the tools you use, it's how you use them. Just like some people have mined vampires, werewolves and zombies recently to really interesting effect... I'm certain that elves... and dwarves, trolls, gnomes and orcs... could be used in a new and interesting way. Heck, Stan Nicholls's trilogy of Orcs books has caught my attention. I haven't read them yet, but I'm intrigued to see what he did there.