As it is now with a selected object I have to press alt (or shift or cmd) first and then drag the object in order to duplicate it. I can even let go of the modifier key while dragging once the duplication has been initiated. This is not Mac-like.
I suggest to change this behavior to work just like the Finder in OS X: I can press alt any time while dragging an element and this changes the drag action to a drag-and-duplicate action. Also helpful is the arrow changing to an arrow-with-a-plus-symbol thus reflecting the modified behaviour.
Likewise I don’t think it is necessary to let three modifier keys do more or less the same trick.
Here’s my suggestion for the other modifiers:
– Shift-dragging would constrain the dragging to 45° directions (but wouldn’t generate a copy, for a copy I’d have to press the alt key too). One expansion to the constrain behaviour of now: By moving the arrow around I could still change the constrained direction to 45° jumps – not just to the one direction I started to drag to.
– Cmd-dragging would create not a copy but an alias or symbol (which I think is on the roadmap anyway).