I've never understood the hysteria surrounding Wicked (and some of the cast members) or the preciousness of some people about how roles should be played. Every actor comes to a role with their own ideas on how to interpret it and breathe life into the character. Also, every Musical Theatre actor has different vocal capabilities, so what one actor does vocally, isn't necessarily going to fit another actor's voice.
Surely it's better to have a series of actors bringing something new and different to a role, rather than have them slavishly copying each other.
As for Rachel, I'm pleased for her that she's got this role. I'm sure she'll give it 100% and I hope she enjoys her time in Wicked. I also hope that the hardcore fans give her a chance and judge her on her own merit, not in comparison with previous Elphabas!