I recently overheard a bewildering comment from a pair of authors. The first one asked:
"Have you done any research for your story?"
Author #2: "I write fantasy! I don't need to do any research! It's all made up in my head! It's not like I'm doing historical fiction or something like that!"
Honestly, I'd thought the same thing thirty years ago, when I wrote my first princess-and-the-dragon story. My English teacher read it, then commented, "Your story is wonderful, but the Dragon can't fly. Not the way you've described him."
"What? Why?" I asked. (Keep in mind, I was eight years old at the time).
"His head's way too large for his body, and your story has a planet with extra gravity. He'd be lucky if he doesn't get his nose buried in the ground when he takes off."
"But it's all made up!"
"It's your world, but even fantasy worlds have to follow certain Laws of the Universe (note the Capital Letters)." She directed me to Tolkien's Lord of the Rings, with so many different faces populating Middle Earth. I spent some time reading The Fellowship of the Ring, and marveled at beautiful Galadriel, brave Arwen, and wise Elrond, awesome archer Legolas and funny Gimli the dwarf. After I finished Fellowship, my teacher asked me how I liked it. I told her I loved it, then I asked a question that I bet no other English teacher has ever heard:
"Why are all the Elves skinny? And all the dwarves short? You never see a chubby elf."
Her mouth dropped open as she tried to figure out what to say. Then she said, "Just because you haven't read about a chubby elf doesn't mean they don't exist."
I promptly wrote a story about Timra The Chubby Elf and her compatriot, Huisai the Extraordinarily Tall Dwarf. Time has made that particular manuscript (scribbled in pencil on regular notebook paper) disappear. If I still had it, I probably would post it. I was writing something different, something against type. In Biology class, I learned about different heart rates and weight ratios and bone structure. My English teacher was right: my dragon wouldn't be able to fly the way I imagined him. And considering an Elf's internal make-up (lightweight bones, heightened reflexes and stamina), there's a reason why you see renditions of them as tall, elegant, and ethereal.
A person from a high-gravity planet would most likely be physically short and stocky, if you take into account the effects on gravity on your bones. Someone from an eternally-dark world should be able to detect objects via sound waves, like sonar, but be almost completely blind in the sun. Gravitationally-challenged Dragons, Chubby Elves and Extraordinarily Tall Dwarves aside, the possibilities for your fantasy world can be endless. Mages, Demons, Spirits, Fairies, Ethereal-Stuff-Between-Dimensions, and your dashing hero/plucky heroine...but be careful not to cross into the realm of Impossible. Nothing else pulls the reader out of a story like that.