I think it's directly related to power. When she was defeated, the elements of harmony took almost all of her power, but by the next episode we saw her in, she had regained it.
I think alicorns' manes are directly linked with their power. Celestia and Luna are the most powerful alicorns, probably because they're 'natural' alicorns, alicorns from birth. Cadance and Twilight ascended to alicornhood, so they don't have nearly as much power, and their manes don't move like Celestia's and Luna's.
Anyway, have a pic of the different forms of Luna.
Another pic of Luna. This one I like particularly because it follows the show style pretty closely, but also adds several subtle touches of its own.