Just got back from seeing this. I enjoyed it a great deal, as did my little one.
It's a bit of a strange Disney movie in that it doesn't follow their typical formula, which is what I probably liked the most about it. I don't know if it's based on a legend or not, but it feels very original which is nice.
There are several songs in the movie which at times makes it feel like more of a musical, but they're decently catchy enough. The movie's themes were just a tad too advanced at times for my 4-year old, so I think the abundance of songs helped keep her interest.
As far as characters go, Anna is a terrific character and a worthy entry to the Disney princess lineup. She continues the theme of the strong independent female ala Merida in Brave, but she also has very human flaws. The audience unfortunately doesn't get a chance to be as close with Elsa, so she'll probably be playing second fiddle in the merchandising.
Olaf the Snowman is one of the biggest highlights of the film, and it's unfortunate he isn't used more. I'm sure we'll see him again in future short features, though.
It would take some time and distance for me to say how Frozen ranks with the best Disney animated features, but the short feature before the movie was a nice preview of what was to come. It was a hybrid of classic Mickey Mouse mixed with modern technology and storytelling. Frozen is the same way - grounded in traditional Disney movie-making magic, but experimenting with moving away from tired stereotypes and storylines.