So literally every person in this thread has screwed something up at some point.
At time of introduction, the M4 was an outstanding tank. It could reliably penetrate the 50mm armor of the Panzer III and IV at ranges exceeding 2000 yards.
It was also well protected, having about 87mm effective thickness of frontal armor on the glacis on the cast hull M4A1 and 103mm on the welded M4A3 (note that this is superior to the Tiger I at close-medium ranges)
But it had flaws; notably ammunition storage and a poor design of the optics, reticulated with a simple ladder sight, and lacking any clear horizontal graduations for target leading. This introduced potential for misestimation of both lead and range as the gunner had to estimate what the gun was layed to based on his own interpretation of the sight.
This was compounded by low shell velocity.
Optical clarity and magnification were good if not outstanding.
The M1 76mm retained the flaws of the gunsight, but with improved 6x magnification and higher shell velocity and a flatter trajectory, reducing the likely impact of human error.
Notable for the 17lber, which had a variable magnification ladder sight similar to the M3 75mm and M1 76mm, however the maximum range graduations extended beyond the field of view at its doubled 6x magnification, potentially limiting the magnification's utility in long distance gunnery (but remaining fully useful for target identification and acquisition).
Additionally all Allied sights lacked any range calculation system.
The vulnerability of the M4 body to ammunition explosions and brew ups was greatly over exaggerated, likely the result of human negativity bias. The wet stowage was a solution to an imagined problem, though still generally useful.
Notable as well, brew ups and fires weren't a tremendous concern for the German Panzers either, nor were the diesel T-34's themselves any less vulnerable to engine fires given a proper source of ignition. Diesel is simply less volatile, and slower burning than is gasoline of any octane or composition. This resulted in more time for the crew to bail out.
Regarding the 76mm and 17lber HE shells; it didn't matter. In virtually all situations, machine guns were more effective, or the target was well enough dug in that the difference between the shell's bursting charge was largely irrelevant. If a machine gun is well sandbagged and dug in, it can survive anything up to a near miss from anything up to a 105mm.
High explosives have a very crucial, but very narrowly limited use on the battlefield. Any competent individual who has seen service or worked with explosives will tell you that a few ounces of blasting compound does not make up for poor placement of your charges
But the M4 was reliable, produceable, maneuverable, and capable. However it was not upgraded as it should have been, and it wasn't without its flaws.
The reality falls somewhere between the two vocal groups on the board. It was far from great, but neither was it awful either.
As for the German weapons, their gunsights worked on a different system entirely. The reticulations themselves were moved as the gun was layed to a different range (as indicated by range graduations around the interior perimeter of the gunsighs), eliminating any need for the gunner to use hold over to aim. Additionally the gunsight had lead markings graduated at 1 mil (apparent size of 1 meter at 1000 meters) increments, doubling as a range calculation system using known length, width, and or heights of enemy tanks, allowing gunners on the 88mm L/71 to achieve 85% first round hit probability under ideal conditions at 2km.
Magnification was average on 50mm and 75mm weapons at 2.5x, and good on 75mm L/70 and 88mm weapons. Optical clarity was outstanding.
Combined with a high shell velocity and flat trajectory, this made quality German tank crews exceedingly deadly in engagements at extended ranges.
Taken as a system, I would say that these all made the KwK 42 L/70 the best cannon of any nation in the war.
Regarding the Panthers weight, as best I can calculate (lacking exact composition or density of German steels, I used density for cold rolled chromium-carbon tool steel as an approximate substitute), the increase in the Panthers armor thickness from 50mm to 80mm only added 3 tons out of the near 10 tons of weight gain over the prototypes.
I cannot account for the rest of the weight gain.
Alright, thread arguments resolved. Case closed. Somebody lock it please.