So, I heard a theory that nicely explains to me why everypony was so frightened of Luna initially. Basically, the visual changes to her were not for viewer benefit only.
The last time anyone in Ponyville saw Luna, it was right after she'd been hit with the Elements. All of her corrupted/corruptive magic was gone, and her normal powers had yet to reassert themselves. As such, she was smaller than Celestia by a wide margin, and had solid, normal hair.
Now, by the time she comes back, she's had over a year to recover. In addition to a year of natural growth, I think that the return of her powers had a physical change as well, making her a bit bigger and making her mane into a starry one, to match Celestia's.
Now, the last thing Ponyville saw was that diminished Luna. So, when Luna returns over a year later, but taller, sporting the ethereal mane last seen on Nightmare Moon, and being pulled by an utterly
demonic-looking chariot, I think they can be forgiven for their first thought being "
Oh Crap. History's repeating itself, and she went evil again!"
Of course, her initial speech
should have cleared it up, but she had to go all Traditional Royal Capslock Voice when delivering it.
I have a message from another time...