They're giving you a bonus if you pre order. It's like a thank you gift.
Oh... How weird
Well in that case, I would think it should be something optional, and not really change anything major about the game. So people who haven't got that copy are missing out on a quest or something, but maybe the option to take an enchanting ring of sorts or a mount (unicorn I guess) available from the beginning. I think I'd prefer something physical like a map or something, which, as pointed out, is more of a collector's edition thing I suppose.