What was different about this film was that the singing was recorded as they were acting. Usually the singing is done in a recording studio separately and then dubbed onto the film afterwards.
So when you see Anne Hathaway singing on screen, she really is singing and it is being recorded at that moment, rather than miming!
THAT is what is special about it.
If she got it right first time, I would be as impressed as I was with the finished product.
NB. Every time someone records a song, they are really singing.