Rhythm Thief stands as a sort of fusion of this kind of "narrative music game" and Rhythm Heaven, combining the persistent storytelling of the former with the wacky variety of the latter, with some of the style of Elite Beat Agents.
Which is a circuitous way of saying "this is really the kind of game I like." I'm not convinced it'll be a true classic like those others, but it has the right inspirations, at least.
The TGS demo consisted of four levels, all with totally different, simple tasks. In one, I was asked to jump from rooftop to rooftop, pressing either A or B in response to a prompt. Another had me dancing (by swiping in a listed direction), following the lead of two dancers before me, such that I had to not only match their moves, but the interval between each dancer's movement. Like Rhythm Heaven's Glee Club. Another intensely amusing game has you tapping either the D-pad or a button to fight attackers coming in from the left or right respectively. The final game of the demo tasked me with tilting the 3DS left or right in time to the music, which coincided with my feeling that it was time to go.
More bluntly, tilting the system with precise timing didn't work and made me angry.
Speaking of things that didn't quite work: the only music I remember from the demo is a goofy song built around an obvious Kool & the Gang sample. I should note that it would be premature to judge the music in this game as terrible based only on one half-remembered song and three snippets that failed to make an impression, heard inside a convention center. My point is, the bits I heard weren't awesome enough to stay with me, and that in itself is a big deal for a music game with original songs.
I desperately want the music to turn out good, because I want this to be great. I'm not sure it has any chance of being a big seller, mind -- music games built on non-licensed music are at a disadvantage, wonderful though they may be -- but, if done right, I'd be more than happy to add Rhythm Thief to the modest collection of such games that I constantly replay.