Personally, I found the view of the younger sister pretty pathetic (basically, needing a man or love to have a full life). Surprised more hasnt been made of that than how the let it go song is about coming out (or isnt).
It wasn't bad. The music was good for a recent Disney film, could very easily be made into a musical. My kids sing it ad naseum. LET IT GO LET IT GO...........
It go so popular because it's quintesentially a movie for little girls. It's not a man and woman in love, it's two friends (sisters) and their relationship. It's more relateable to a little girl than a prince marrying them.
I thought the whole finding love with the aristocrat and then with the ice cutter was a huge part of the movie. There were about 20 minutes total of interaction between the two sisters.