I think I'd have to agree that one of the biggest effects on fanfiction from the films are Snape's looks.
I can't even count how many times I've come across a Snape saves Harry from the Durlseys fic and the author's put Snape in leather trousers riding a motorcycle. It's ridiculous. I just saw a fic the other day as well that had Snape walking through Muggle London and attractng quite a bit of female attraction because apparently he was rather fetching. The ONLY time I've ever seen Snape attract the attention of a female was when he was being strung up by his ankle and his graying pants were showing. Because Alan Rickman isn't exactly an eye-sore (though he makes a fantastic Snape), doesn't mean the character isn't drop-dead ugly.