I'm a huge
Luna fan. I think people liked her because she was so different. She cast a special light on the HP crew. She spoke the plain truth in a world where everything was secretive and when
Harry could only learn things when it was the perfect timing for Dumbledore (and plot's sake). She had a thought and she said it versus the millions of other characters who kept everything inside until it was appropriate to say.
She marched to her own drum and I think almost everyone, if not just a lot of people, consider themselves weird. And they like the underdog. So here was a girl who was clearly weird and clearly the underdog so people just latched on. Plus, she was entertaining! At least, to me. :)