It's a Disney CGI animated film inspired by Hans Christian Andersen's The Snow Queen. Six months after release, it passed a billion dollars in global box office and because the highest grossing animated and musical film of all time, not to mention also the ninth highest grossing film of all time.
Is it good? Answer: yes, don't be stupid. In the end it's brilliant, although it has some minor problems with the CGI animation, the Hans character and the handling of the songs. I wouldn't put it above The Little Mermaid and Mulan, but it's giving them a fight.
I'll be mentioning a big spoiler, by the way. You have been warned.
So, what's the plot? Elsa is a little girl cursed with ice superpowers, like Iceman from the X-Men. One day she nearly kills her little sister Anna and as a result their parents wipe Anna's memory, ban Elsa from ordinary human contact until the day she can control her powers and then inconveniently get themselves killed at sea. (You might be wondering about the memory-wiping. Answer: the film's setting is based on Norway and on Scandinavian culture in general, so it has cute magical trolls.) Anyway, years later, Elsa has come of age and about to ascend to her late parents' throne, but unsurprisingly she hasn't learned much about anything while stuck in a palace on her own.
However there's still that sister. Anna loves Elsa, even though she's barely seen her for years. A couple of boys will end up in the mix too, which makes things unpredictable in the same way Jane Austen managed in Sense and Sensibility. A Disney film with one boy and one girl has been railroaded. There is zero uncertainty about their romantic prospects. Two boys and two girls, though, allow many possible happy endings, especially in a film that's being interestingly flexible with its hero-villain boundaries. Elsa's the Snow Queen. In a different Disney film, she'd be Cruella de Vil... and indeed she was a flat-out villain in many drafts of this one too. Giving her a sister created a largely unexplored dynamic in American animated films. We've all come to expect a Disney princess to be looking for her prince. This is much fresher and more empowering, while of course it's also speaking much more directly to its child audience.
Act One has problems, which I'll get to. Act Two is very good. Act Three is magnificent.
There's no mystery about why it's been world-conquering. I think it's superb, so what didn't I like about it?
Firstly, the CGI, even though in many ways it's superb. It looks beautiful and it has clever, witty touches (the painting-poses, the cliff-jumping snow silhouette, the trolls blinking in unison, the icicle telescope, etc.). The face acting is also so subtle and strong that it must have involved with CGI performance capture technology. It's no different to Star Wars or Lord of the Rings, except in a different rendering style. (Even that's attractive, by the way, with a softness that avoids the shiny plastic look you often get with CGI.)
On the downside, though, this makes the animation stiff. The performance capture and the mathematical reality of computer-built imagery ties the film down to literalism in a way that never happened with hand-drawn animation. Just a little bit. A touch. Here and there. However, every so often, this falls flat. "She's my sister; she would never hurt me" is weak, for instance, while the biggest problem is the musical numbers. The transitions into song can be almost painfully bad, after which their choreography can be ugly and the actual line delivery is liable to suck. "Okay, bye" in 'Do You Want to Build a Snowman?', for instance. These people can sing and act, but not always at the same time.
As it happens, most of the songs are in Act One. This is part of why the film improves over its running time, although in fairness the film strikes gold with the Oscar-winning 'Let It Go'. (My sister was on a plane recently and Frozen was among the movie options. Everyone knew when the film had reached 'Let It Go', because seemingly every little girl on the plane was singing along.)
The other problem is Hans, who's underwritten in act one and has a character-killing moment of stupidity in act three. Admittedly it's mildly refreshing to have a girl at the centre of the story and for the boy to be cardboard, but that doesn't change the fact that the guy hasn't really done anything except be gorgeous. Later, in hindsight, we can read between the lines and see some subtleties in Hans's characterisation (e.g. the way he mirrors whoever he's talking to), but then how stupid is he when being evil? Why didn't he just kill Anna? He could have easily finished her off in a couple of minutes, or at the very least waited to watch her die before announcing the tragic news. It's not as if he's not arranging Elsa's murder too. That would have guaranteed his victory and averted the entire third act.
The answer to this is of course "it's a Disney movie". Also it's not technically a plot hole, because he thought she was dying and so we're really just criticising his judgement. Nonetheless it would have been easy to make even a Disney-friendly version more convincing, e.g. make the dying Anna look like someone who's actually dying, instead of merely earnest and determined.
Tomoko didn't like this film, incidentally. She had no patience for Elsa's angst and thought Anna an idiot for a certain bubbleheaded action. Personally I think that's harsh, but it's true that the childhood accident at the beginning is Anna's fault, not Elsa's. Their parents' assumptions are wrong-headed, although understandable, and the whole film is basically the collateral damage of over-protective parenting. If your child has superpowers, ensure they grow up into the perfect adult by making them live in isolation and brainwiping their siblings! Tomoko thought them idiots and approved when they died.
None of all that comes even close to killing the film, though. I'd have also liked less cuddly trolls, but hey. It's Disney. The film's pretty good even at the beginning, but its third act is gold-plated, making impressive use even of comedy characters like Olaf the snowman. It does cool and unexpected things with a hoary old Disney staple ("an act of true love") and in the end it's a movie to make you stand up and cheer.
Compared with all that, its problems are trivial. Most of them would disappear if you did it in traditional hand-drawn 2D, for a start.