Personally I think the biggest dismay is caused because Nightwish after Tarja is different. Naturally. Marco gets a lot more space as a singer and musician that he didn't get with Tarja, you get to hear a lot more sound behind the voices, plus the music isn't centered around the lead vocal (my impression). They had grown, evolved. Maybe that just came with the years passing and it's hard to say whether that would happen with Tarja too, but somehow I doubt it.
Some people like the change, some people don't, but there are really only few who are somewhat indifferent.
This way or that, Tarja isn't coming back. Tuomas makes sure to be damn clear about it and I must say, Anette has my admiration for not running away from all that bashing.