When looking at today's logic ICs, for example CPUs, those seem to consist of a very thin and dense layer of transistors, wired by a large amount of "metal" layers on top to complete the circuit.
For flash memory chips, on contrast, it seems that the memory cells can be stacked to several layers on top of each other.
So with all the never-ending need of scaling logic circuits, why not just back up a little on dimensional scaling, and stack the transistors on top of each other? I first thought of maybe thermal reasons - it is already hard to cool the single layer we have today - but more transistors may be clocked with lower frequency, lower voltage and thus lower power dissipation. Also a 3D-layouted circuit will greatly benefit from shorter wiring, thus it may be inherently faster and lower power.
So what is the show stopper on stacking circuits on logic ICs ?