My politically incorrect views:

Wicca is a made-up 'religion' of which 90% of it's adherents are middle and high school aged kids, mostly girls, who are rebelling against their parents and christian upbringings. The other 10% are people who've made it past high school and still for some reason believe in it. It's a very 'fashionable' 'religion' for younger kids in that age bracket.

As Adrian said, she's compensating. I'd posit a slightly different angle, and that is she's rebelling. All kids generally do this to various degrees in that age period as they find their own identities.

ATPG, there IS no difference between it and other religions.

Meneldil. Best thing I can say is just don't feed the beast. She's probably going to be hard and heavy into this for a year or so, that's about how long it lasted for my friends in high school. I don't think anyone made it past freshman year caring about it, because usually by that time we'd discovered new freedoms, and with them gods to worship (drugs, alcohol, cars, partying, etc). If you want to stay with her, you'll probably end up walking a fine line between not giving a rat's ass, yet indicating to her that you understand it's important to her, but you don't share her views or agree. Or something like that. Or just dump her and go date one of the hot cheerleaders.

Ahhh... high school. So long ago.