Oldish thread, but I have kind of an odd perspective about this debate.
I don't like drama within bands, and members getting kicked out. I know it happens, and sometimes bands just really cannot function with certain people in them, and often it's for the best...but eh, it's just something I don't like.
(As it happens, I managed to completely miss the Nightwish/Tarja drama while it was going on. My first inkling that anything had happened at all was listening to The Poet and the Pendulum for the first time and going "Hold on...who the hell is THAT?" I...don't keep up with band news all that much.)
That said, when I'm actually listening to the music, I tend to get so wrapped up in the story within the lyrics and melodies that I don't even think about who's playing. It's one of the reasons I love Nightwish so much...their songs are such intense, thrilling stories one can get lost in. (I write fantasy novels and Tuomas is one of my primary muses.) Not only does each Nightwish song have a story, but every album is also a larger story and there are themes that run through every album, that get taken in different directions each time.
So I see the transition like this:
Tarja was the voice of the goddess, singing for the child. Beautiful, ethereal, magical...but somewhat distant and at times, cold and maybe a little hard to wrap your head around. Anette is the voice of the child herself, come of age. Sweet and gentle and maybe not so "perfect", but I love that elusive quality of innocence she brings out.
You can't compare one to the other...they each have a place in the overall arc and both were, in a sense, necessary to the story.