Some say Tarja's voice made Nightwish what they are, other's say that Tuomas' songwriting makes the band special. There are an enlightened bunch that say it is a combination of the two, or the band as a whole. Me, i lean heavily towards saying "chiefly Tuomas, but the band as a whole".
From a *musical* standpoint, i don't imagine there are many fans out there (out of those who knew of/enjoyed Nightwish before DPP) who would say they prefer Anette over Tarja, but most of us appreciate that Anette can sing. I admit, at first she sounded a bit generic to me too, but as i became more familiar with her style, i no longer thought that way, and really, let's be honest here: Many people would have never been satisfied with *any* replacement. Not a soprano (which they would assuredly call a "pale imitation"); not even a choir of angels sent down from some heaven. So really, there would have been simply no way to satisfy some fans. Also, all my experiences thus far suggests that she is a fair sight friendlier and more sincere than Tarja (at least concerning regions that weren't immediate cash cows, like North America), and that goes quite a long way. But i don't want to get into a "which one is better" debate. They both have their strengths and weaknesses. We can leave it at that.
Suggesting that Nightwish with Anette is no better than Evanescence is completely dismissing the songwriting ability of Tuomas and the rest. Not to mention that it comes dangerously close to marking you as a troll. We generally try not to bring up Evanescence anyway, those conversations go nowhere, fast (and have been beaten well into the ground anyway). Regardless, most of us will readily agree that DPP is hardly their best album. But it's still a good album, one that i am more than glad to have.