NON-SPOILER ALERT: I'm not going to spoil anything beyond the first 20 minutes of the movie or so.
The first thing I have to say about Frozen is that the promotional campaign SUCKED. The ads and trailers give you ABSOLUTELY no idea what the movie is actually ABOUT, make the movie seem shallow and mediocre, and could even be considered a bit misleading since they put the focus on things that actually prove to be fairly minor. (For example, Olaf the snowman is NOT a meaningless, shallow comic relief character; he's actually important to the plot, not annoying at all, and he sees character development through the course of the movie.) From the trailers alone you wouldn't even know the movie is a musical in the tradition of Beauty and the Beast rather than just having a couple musical numbers thrown in like Tangled did.
In fact, the movie is actually pretty deep, with some really surprising twists that toy with your expectations of cliches.
Since the ads don't actually TELL you what the movie is about... and in fact the only plot information I can find about the movie outside of the movie itself is
http://movies.disney.com/frozen/story ... I'll give a bit more information.
Elsa and Anna are sisters, princesses of Arendelle. Elsa is the older sister, and heir to the throne; she was also born with ice magic that she struggles to keep under control. An accident claims the lives of their parents, and when Elsa comes of age she is crowned queen. The emotional stress on the day of coronation makes her lose control of her power and she flees the castle to seek solitude, but accidentally freezes the entire kingdom in the process. Anna sets out to find Elsa to get her help in restoring summer, and to bring her home.
By the way: The soundtrack is AMAZING.