This is Julianne Moore's film
The impact of Nicole's performance has drastically receded with the years and although Meryl's performance still holds up very well (she is very believable), this film belongs to Julianne Moore. She is the soul of the work. Without her, the film would be about Kidman's tortured genius and Streep's lovelorn, pensive neurotic. Only with the addition of Moore's depressed, frustrated romantic does something click with this film IMO, truly bringing it down to earth.
PS - by the way, as far as Nicole's segment of the film goes, the performance that really engages me there is Miranda Richardson as her sister Vanessa. Made me wish she could have reprised the role on stage or in another film from Vanessa's perspective. Such a spirited eccentric character (note the way her kids refer to her as Nessa instead of Mother and how she refers to them as "changelings")