Personally speaking, I don’t think the animation in Frozen was anything particularly special, at least in regards to other major releases from Pixar, Dreamworks, or even other recent Disney films like Wreck-It Ralph. That’s not to say it isn’t good; quite the contrary, pretty much all three of those studios put out consistently excellent films. That said, besides the aforementioned avoidance of the uncanny valley, they did do a good job of creating a consistent aesthetic.
What I mean by this, to use video games as an example, is how some 2-D games on the SNES, as old as Super Mario World still look good, whereas some games even as recent as a few years ago don’t. Realizing the limits of the technology is an important aspect of the character design, so that they can be designed to make the most of it and hide the flaws, but particularly avoid showing things that either create breaks in that consistency or compare them to things that they know will improve as time comes.
Consider a film like the original Toy Story. For the most part this film holds up remarkably well for the same sorts of reasons, despite that the technology is much older. However, if you rewatch it, as soon as the kids show up on screen, it looks horribly dated.
That’s one of the awesome things about modern animation is that, if it’s done well, we notice it looks better as things like sub-surface scattering, or more realistic texture effects, particularly for hair, are improved, but it’s usually in a way that we can “feel” but not particularly identify. This is something, I think, that Frozen did well, aware that the uncanny valley is still very much a real thing, they designed the characters around that with much better hair and skin than we’d seen even a few years ago, but were still distinctly animated enough to stay on the good side of that cliff.
The problem is, no matter how good CG gets, we can still always notice it for people. Consider how easily the CG apes disappeared in last year’s Rise of the Planet of the Apes, but even in a film like this year’s Avengers or the recent Hobbit films, there’s something a little off when they CG something that looks more human. Animation can make aesthetic choices to avoid showing more human characters, live-action has a harder time doing it and often has to result to odd color palettes, lighting effects, pan smears, fast cuts, etc.