"Let It Go" is for me a very great and fabulous song.
I heard it twice at some private presentations of Frozen in France with an unfinal music background (I hope some instrumental parts didn't change though because there's a part with a piano I adored!), which brings a lot of depth and magnitude.
The entire song might be 4 minutes, maybe a little less.
I remember that everyone at both presentations was amazed by this song and that most of us thought it was one of the best Disney songs for a while.
Not to mention that if you add the visuals (animation etc...), it makes a key scene in the movie that is memorable, especially when I was lucky enough to see the previous scene, so it helped a lot to be IN the movie and feel for Elsa.
Oh, and sure she says "the past is in the past" at some moment. It is the culminating part I would say and Idina Menzel is amazing, because the song is going crescendo, a bit like "Defying Gravity" in Wicked.
I also remembered that Elsa's acting and way to move in this song is particularly brilliant and close to Idina, so I can understand and visualize the diaphragme thing.
The doll song version seems a little flat to me and frustrating as I know the full song by heart and heard it several times...