Rincewind, I agree with pretty much everything.
Using tap-and-hold as a universal indication of the start of a gesture is perfect; I hadn't realized you included that as universal. Also, I like having all gestures produce the visual cue (a line on the screen).
I think that adding a little icon in the corner wouldn't be TOO hard to do, but I'll look into how to do that and implement it myself in v2 or v3.
This is because, if the default behavior when NOT gesturing is to scribble, how would a user know whether they had successfully started "gesture mode" or not?
It occurs to me that there might be two paradigms for how gestures might work... one is, you make a gesture (i.e., draw a Z) to start zoom mode, then you actually draw the zoom box. The other paradigm is, the zoom box is part of the gesture (i.e., press and hold, draw a box, come back to where you started and hold, release and it zooms). It seems like enabling both paradigms would be more complicated...