Well, as for unintuitive I still don't know how. Let's say I have a stack of pickaxes. I enter the inventory, filter on tools, press the letter corresponding to the pickaxes, then press 2 for 'some'. Is that how it's supposed to work? But then it drops all tools, and the only difference is that I get dialogs for each stack asking how many. Don't get me wrong, the new interface is definitely more powerful and makes more sense, but so far that's one use case that I haven't grokked.
As for the cancel, is it possible to just have the Z option in each dialog in order for to abort the rest of the drop action, if they made a mistake?