Size and Weight:
In order to understand the importance of Xiaotingia, you need a short lesson about a much more famous creature, Archaeopteryx. When the exquisitely preserved fossils of Archaeopteryx were discovered in Germany's Solnhofen fossil beds in the middle 19th century, paleontologists identified this flying, feathered creature as the first true bird. That's the image that has stuck ever since in the popular imagination, even though better-informed naturalists know that Archaeopteryx presented a weird mix of bird-like and dinosaur-like characteristics, and probably should have been classified as a feathered dinosaur (rather than a bird) all along.
So what does all of this have to do with Xiaotingia? Well, this very Archaeopteryx-like critter, discovered in China's Liaoning fossil beds, predated its more prominent cousin by five million years. More important, the team that discovered Xiaotingia identified it right off the bat as a small, "maniraptoran," theropod dinosaur rather than a prehistoric bird, the implication being that if Xiaotingia wasn't a true bird, then neither was Archaeopteryx. This has caused huge shockwaves in the "Archaeopteryx was a bird" camp, but hasn't impressed those more dubious paleontologists who doubted Archaeopteryx's credentials in the first place!