I was watching this interesting documentary in Discovery Channel about tanks, mainly which 10 ones are the best in history. It had a segment on the Panther tank. In that segment, they showed a team whose project was to restore a Panther tank to mint running condition. During the course of overhaul and restoration, they discovered that many of the parts used in the Panther are vastly over engineered, strong enough to last for years of abuse. Yet the tank is only expected to last for weeks in the battle front.
That obsession for over engineering probably helped did the Germans in. They continued to insist on building things that were too complicated or in quality levels that wasn't necessary for their predicament.
In the course of the show, the Panther was duly restored and a former Panther tank commander was given the honors to give it a whirl.
They also showed the T-34, and a certain rich person (I think he was a rock star) showed his running T-34 tank, which he had a great blast driving out in the range. The T-34 was quite agile for the concept of a tank, and remarkably drivable. It seemed idiot proofed enough for any Russian peasant boy to get in and drive it quickly.
Despite the lackluster optics of the tank, it didn't seem to matter, since a lot of T-34 battles were duked out in close range. A tactic that tank frequently uses is to ram the enemy, where the wedge shaped end of the sloping armor in the front of the tank makes a great bumper. In the course of the war, they also explained that many T-34s were simply driven out straight from the factory and into the war front without any need to go through quality inspection.
Back to the original question-
It was not just good-quality tanks that allowed the Panzer Divisions to succeed. Everybody had tanks back then.
In 1940, the Germans also had armored Halftracks to carry the Infantry working closely with the tanks, and other Halftracks with Flak guns. There were also Self-Propelled artillery units; Mk. I's with 150mm Infantry cannon. Also assualt-artillery, fully armored chassis with 75mm guns meant to face strong enemy fire... and press the attack.
Nobody else in the world had vehicles like these at that time. They existed because of good planning and serious staff work.
Very good. And yet there was something the Germans didn't build enough of, a vast oversight if you will, which tends to happen on things and matters people would consider mundane and that helped do them in. That vehicle is the plain old truck.