One thing that impressed me about Enchanted was the near-effortless way the filmmakers managed to carry off the live-action musical numbers. Back in the '90s when nearly every animated film was a musical, you sometimes heard people say that a cartoon was the only place where you could "get away with" doing musicals. Unfortunately this often became a justification for doing projects in animation when they'd probably have worked better as live-action musicals. But Amy Adams' two big numbers in Enchanted are live-action set pieces that work perfectly well even though the film is more of a comedy with songs than a full-fledged musical (Adams ceases to be a singing character once she becomes a fuller part of the non-animated world). If a live-action director creates an atmosphere and a setting where people will buy into a song and dance number -- in this case, of course, the atmosphere is that of a Disney cartoon musical in live action -- people will accept it.
It is true, though, that audiences tend to accept singing in a cartoon more readily than they do in live-action. That is, a live-action musical has to do a lot of work to be sufficiently stylized and heightened before the audience will sit still for a musical number, whereas the same audiences just accept it in a cartoon because it's a cartoon. There are a number of possible reasons for that, but I suppose the most obvious reason is a cartoon world is inherently stylized and heightened, so it somehow seems instantly plausible that moving drawings would sing. Remember that while early animated features were mostly musicals, 3-D animated films never had that kind of expectation: there's just something about the more "real" world of computerized animation that doesn't lend itself to the expectation that characters will sing.
Also, animation is rhythmical -- frequently drawn and timed to music or at least a beat -- so the beat of a song can sort of naturally rise out of an animated scene.
I think that in the last few years, 2-D animated features have probably gone too far in avoiding musical numbers. This was an understandable backlash to the '90s, when animated features went too far in the other direction. Songs in an animated movie tend to work best when no one thinks of them as an intrusion; the songs in Snow White are so short and perfectly integrated that you can watch the film without being fully aware that it's a musical. And while the Menken-Ashman songs were more elaborate and Broadway-ish, they still came off as natural outgrowths of the scenes. But a lot of the '90s animated features, at Disney and elsewhere, were movies where you were in fact conscious that they were stopping for a musical number. Whether the song is good or bad, if a movie ever stops, it's in trouble; animated movies need to have momentum, because if the audience ever gets bored then they stop thinking of these drawings as actual living characters.
But songs do add a lot to the classic animated features, and I think that audiences really do like the songs and have an expectation that animated characters will sing. Not all animated characters, but certainly fairy-tale characters, magical characters and the like; a movie that takes animation and adds fantasy to it somehow seems so heightened that it almost cries out for singing. (Whereas with a more realistic subject, like Bambi, I don't think singing is expected or needed; that's why Disney wisely decided not to have the Bambi characters sing.) One reason for the success of Enchanted is just that animated fairy-tale musicals are so enduringly popular. Since there hasn't been an animated fairy-tale musical in god knows how long, they'll take a live-action homage to animated fairy-tale musicals.