The TD looked like a tank, but it wasn't. It has a large gun for taking on enemy tanks, but was very lightly armored and lack overhead protection for the crew. The light weight was its only real defense; the idea being to make it fast and nimble enough to evade enemy fire.
At the time, there were several different varieties of tanks, for reconnaissance, for quick breakthroughs of enemy lines, and to support infantry on the ground. But by and large, the tank's main job was believed to be infantry support. American tanks for this role were usually given a medium-strength armor suite and a medium-range, low-velocity gun. The M4 Sherman is the best example of this.
It was a good infantry support weapon, but it was terrible for going head-to-head with enemy tanks. As both tanks and tank destroyers were knocked out by enemy armor in huge numbers during the war, designers began to take the features of the TD and graft them on to the tank, creating an good infantry support vehicle that could theoretically take on enemy tanks, as well. This lead to the Main Battle Tank concept we have today.
But a lot of men died because because of that disastrous misunderstanding of the role of armor on the battlefield.
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