Here is my 3 cents.
I like the dark ending, what I didnt like is at the very end everyone came back on as if no one had died (i dont mean curtain call, I mean just before the end) this gave the impression that the whole show was to be viewed as being told by a bunch of actors, as opposed to trying to represent an event in real time, this is fine, but they do not establish this in the begining, so it just looked odd to me to see all these dead people come back on as if nothing had happened.
I understand the comments about keeping the ending, but honestly I prefer the darker more realistic ending.
as for the design, as groundbreaking as it was, I am 100% sure if they decided to bring it back it would have a totally different creative team. As was said, there isnt a theatre on Broadway that can accomodate that set, asside from the raising platforms, there were ledges to the side, this needed a very wide stage. The New Amsterdam has a rather narrow almost square shaped pros-arch. the only 2 theatres wide enough for that design would be the winter garden and mamma mia isnt going anywhere (although I am not sure if the winter garden would have enough room under and above the stage) and the gershwin.
http://www.flickr.com/photos/27199361@N08/ Phantom at the Royal Empire Theatre