Well, the specs are simple enough; a good implementation would let you set the thing flying, then when you touch it, would quickly (but not quite instantly) brake, and wouldn't take off again if you lift fairly straight off.

The latter is the typical problem; it's exacerbated by the nonlinearity and assymetry of a finger-screen interface. When you lift your finger straight, the centroid (as measured by a touchscreen) moves. Appropriate magic to disregard this motion and catch all deliberate flicks without feel degradation is difficult, to say the least.

But my biggest complaint is the same as for kinematic scrolling; the scrolling area usually overlaps the interaction area, so a drag thought better of becomes a click, and a click smeared a little becomes a drag.