Besides the event flooding mentioned previously, the built-in Android GestureDetectors leave something to be desired. In the revamped Firefox Mobile that's under development we're noticing all sorts of bugs (e.g. http://bugzil.la/706684) that are related to gesture detectors malfunctioning, and we already reinvented the kinetic panning gesture detector because we were unhappy with the responsiveness of the built-in one (it does too much rounding).