Picked up the new Detolf on Friday and did some measurements today.
Shelves on both old and new are nominal 4mm (3.91-3.92mm)
Doors on both old and new are a nominal 5mm (4.76-4.85mm)
Sides on old are a nominal 5mm (4.75mm) and on the new a nominal 4mm (3.93mm)
Metal ladder rails on old are 6.09mm and on the new 5.77mm
IKEA seems to have moved to different suppliers for the parts in their new Detolf. I don't have any details for country of origin of the old, but the new ones are made in China.
As I'd already built it, I didn't get to measure the back panels, but I expect them to be the same thickness as the sides. Overall the near 1mm reduction on three sides, plus the reduction in steel can account for some decent bill of material and shipping savings especially when you combine it with streamlining other parts (feet and hole-plugs are now the same across all color variations). Overall, in terms of robustness and build quality, the new model is close enough to the old with the most significant difference being a little more propensity for side to side sway. I think it's a good trade-off for a $20 savings (in Canada: $80 old, $60 new).
I strongly recommend that the top of the Detolf absolutely always be anchored to a wall without exception. If you plan to put the Detolf on top of another cabinet to raise it off the ground (highly recommended) then the bottom cabinet should also always be attached to the wall and the bottom of the Detolf attached to that cabinet/stand.