All the characters have fatal flaws; it's what makes them human-- er, well, Pony, anyway. Moondancer's is being slovenly, sarcastic, and a bit standoffish. Twilight tends to be a bit too down on herself, but she's getting better.
It's a breath of fresh air. Think of all the comics and books, tv shows, etc, with PERFECT characters or characters who just don't seem real at all.