Hm.
Man, I don't know what everyone's talking about; I, for one, loved this game, it's better than all those other Rock-Out games (you have my vote, at least), because, let's face it, the others have the one thing really going for all of them: some cheap imitation of Guitar Hero.
I've never played this game, by the way, this Elite Beat Agents...in fact, I don't even own a Nintendo DS (nor do I support Nintendo in any way, after Super Mario Sunshine...ick. Morals.), and maybe that's why I found this game so much fun, especially the last song.
From what I hear, though, it's supposed to have some kooky graphics in the background, and, just to piss you off, I'll say it, too. This game would be better with them (don't feel bad, though, I believe I'm empathetic your situation...).
The beats, from what I can see, are a little off, but that's just because I've been a percussionist all my life, and we pick up on the slightest things in the world. I didn't find it nothing to be too stressed out about, like some people on here, and I know how hard it is to match up a tempo of 120+ with only 24 (or 12, or whatever) frames/sec, so I'm empathetic about that, as well.
I do hope you'll release a full-er one, use this as a beta or something, and build on it. I had a blast with it, for the ten-or-so minutes I played (the songs are a bit repetitive).
If I were to suggest anything, I'd say J-pop or J-rock (just because it seems so fitting, although I understand the confusion over the music), and, seeing how it's already "stealing" ideas, as one so eloquently wrote earlier, if you really felt like adding background animation, just take a few sprite videos and toss 'em all together for the backgrounds.