Renesmee is truly a unique child. A vampire/human hybrid, her heart beats faster than that of a human, while her temperature is similar to that of a werewolf. She can digest blood as well as human food, but her favorite is human blood. She not only grows and learns at supernatural speed, but she can also communicate by sending images into another mind through touch. Although everyone who meets Renesmee takes to her instantly and spoils her like a child, Meyer describes her as "a grown-up in a baby's body."
That's also why the author voiced her concerns over bringing Breaking Dawn to the big screen:
When I said that Breaking Dawn might be impossible to film, it's because of Renesmee. You can do almost anything with CGI these days – realistic dragons and dinosaurs and endless amounts of nonexistent creatures that blend right in with the real elements. However, the one thing that I've never seen is a CGI human being who truly looks real. An actress can't play Renesmee, at least not when she's a few days old; she's the size of a baby, but her expressions are totally controlled and aware. She would have to be a construct, and CGI isn't quite there yet. Of course, they develop amazing new technologies everyday, and we've got a little time left. (source)
There's also been lots of debate about Renesmee's strange name. What do you think of it? Does it fit?