Not bad.
The game takes a bit of getting used to, but I've made something with a similar control setup once (though not with Flash), so I guess I can't say much. But there's certainly room for improvement.
The graphic style works very well, but the large flashing text makes focusing on all the little people kinda hard. With all the sound cues you use, you could have made those a lot less intrusive and the player still would have known they did something, providing the sound was on, and, if it wasn't, it would be because of that "Nice!" playing over and over again. Add at least a couple other clips in there and randomise it a bit and it'd be a lot less annoying, despite the fact that the clip itself isn't bad at all. It's just how often that plays that makes it annoying.
As for the control scheme, Practice mode was a nice touch, but perhaps on lower difficulty levels, the player only needs to keep track of a couple different keys until the level gets up high enough? And then for each mode higher, you just skip ahead enough levels or some such method to add in that third or fourth key. As a rough idea.
But, as I think I stated, the game isn't bad. In fact, I kinda like it, and would love to see an improved sequel, mayhaps.