The difference in the T-34 armor was that it was sloped. The degree of the slope made the T34 very good as far as protection even though its armor may not have been as thick. Sloping armor even though thin can still protect just as good as thick armor. The germans produced the panther based off of captured T34's. Look at a panther and you see what the germans took from the T34. Now back to the sherman, the original M4 was no match for most german tanks fielded after 1943. The Panzer IV, as is in this game has the 75mm gun and it could take out a sherman easily but the sherman if it hit the panzer it was dead to. But the Panther and tiger could take a sherman hit and keep on truckin. The Easy 8 models had (without researchng) the same 76mm gun the M10 wolverine had and it was a good AT gun but armor was still not comparable to german tigers and panthers. The T34 was a good tank because it has speed, sloped armor, and after the T34/85 came out it had a decent gun, though not as good as the 88 or the german 75mm on the panther which had a high muzzle velocity.
As MiloMorai points out, the armor on the front hull of the Sherman is also sloped, and gets a similar benefit.
Furthermore, the slope of armor certainly helps against smaller caliber weaponry, but is less effective against larger caliber shells. The same 37mm AT shells that so famously bounced off of the T-34s would have also bounced off of the Sherman -- although, unfortunately for the Americans, they encountered few weapons of that size vs. what the T-34s faced in '41-'42.
Take a look at
this link (and hopefully the translator works because the original site is Russian). It is an interview with Soviet tanker Dmitriy Loza who wrote a book about his experiences using Lend-Lease Shermans in WWII. It is quite long, but well worth the read. Note his experiences and impressions of the Sherman and how sharply it contrasts to much of what you hear. A few of the relevant quotes:
Regarding the tendency to "cook off":
For a long time after the war I sought an answer to one question. If a T-34 started burning, we tried to get as far away from it as possible, even though this was forbidden. The on-board ammunition exploded . . . When a Sherman burned, the main gun ammunition did not explode. Why was this? . . . Because our high explosive rounds detonated and the American rounds did not? In the end it was because the American ammunition had more refined explosives. Ours was some kind of component that increased the force of the explosion one and one-half times, at the same time increasing the risk of detonation of the ammunition.
His impressions of the armor:
I want also to add that the Sherman's armor was tough. There were cases on our T-34 when a round struck and did not penetrate. But the crew was wounded because pieces of armor flew off the inside wall and struck the crewmen in the hands and eyes. This never happened on the Sherman.
Regarding whether the US was slow to recognize any problems:
In general the American representative worked efficiently. Any deficiency that he observed and reported was quickly and effectively corrected.
And possibly the most relevant from the beginning of the interview:
When someone says to me that this was a bad tank, I respond, "Excuse me!" One cannot say that this was a bad tank. Bad as compared to what?
This last quote of his sums up my feelings exactly. The primary complaint that the Sherman was a "bad tank" centers around the fact it could not compete against the German heavy tanks. Expecting any 30 ton tank to stand toe-to-toe to the 45 ton Panther or 55 ton Tiger is foolish -- but it does not mean the tank itself is "bad." It means it is a medium tank going against a heavy tank with predictable results. If your standard criteria for whether a tank is "bad" is whether it can fight toe-to-toe with a tank 50-75% larger than itself, then you will also have to relegate the T-34, PzkwIV, and every other medium tank to "bad tank" status.
