I would suspect one of the reasons that we do not have "light" tanks is that pretty much any airplane with a cannon could kill it easily.
+1 though, I still want em.
I just checked the armor differences between the Pzr IV and Pzr III:
Pzr III = 12mm on turret top
Pzr IV = 10mm on turret top.
Pzr III = 17mm on hull top.
Pnr IV = 12mm on hull top.
THe Soviet T70 has 10mm on both turret and hull top.
Interesting finds.