The great Tzaddik, Rav Elazar Abuchatzeira, זצ״ל, was bothered by the severity of the חטא העגל. The sin of the "golden calf" is viewed as one of the worst sins committed by the Jewish people. It is on par with the spies and the selling of Joseph by his brothers.
It may even be worse as the Kabbalists claim that death would have been abolished had it not been for חטא העגל, as the Jewish people on Mount Sinai reached a level of purity equal to that of Adam before his sin.
Rav Elazar felt that perhaps the people were not to blame. They were only free men for seven weeks. And Satan confused them by suspending what looked like Moshe's casket, convincing the עם that Moshe died.
Despite all of this, the seriousness of the sin was the lack of loyalty to Moshe. Everything he promised, he fulfilled; the ten plagues, the splitting of the Red Sea, the great wealth, etc. And now after a six hour delay, the nation turns on him and worships a golden calf. Rav Elazar said that it was this lacking in basic loyalty that made this sin so severe. A very important lesson, indeed.