In this Parsha, the Torah tells us of how Bnai Yisrael responded to the request of Hashem (through Moshe) for materials to build the Mishkan. The Torah says: "ויבאו האנשים על הנשים כל נדיב לב...," "And they came, both men and women, as many as were willing-hearted..." (שם ל"ה:כ"ב). Bnai Yisrael brought all of the materials necessary for the construction of the Mishkan, eventually bringing so much that Moshe had to tell them not to bring any more ('שם ל"ו:ו).
On the surface, this seems very good: Hashem, through Moshe, requested materials for the Mishkan and Bnai Yisrael responded enthusiastically. However, one authority quoted in the Talmud Yerushalmi in Shekalim (פרק א' הלכה א', דף א:-ב.) says that Bnai Yisrael's reaction was not as great as it seems. Rabbi Yehudah Ben Pazi thus says that we must read these Pesukim and shudder because for a good cause, every willing-hearted person participated, but for an evil cause, all the people participated. Rabbi Yehudah thus compares the giving of materials for the Mishkan to the giving of gold for the Golden Calf. He says that for the good cause, namely, for the Mishkan, only the willing-hearted person brought, meaning, only a subset of the people, whereas for the evil cause, that is, for the Golden Calf, everybody contributed, and even broke off their jewelry.
Professor Nechama Leibowitz says that when the people "broke off" their golden ornaments to donate them for the calf, it symbolized a "breaking loose" of the bond of the kingship of Heaven. All the people took part in this "breaking free" experience. However, when it came to the challenge of contributing to the Mishkan, which called for the acceptance of the yoke of Heaven, not everyone participated.
In the Midrash, however, an entirely different, and much more favorable, picture of the giving of the materials for the Mishkan emerges. Commenting on a Posuk in Shir HaShirim which links blackness with beauty (א':ה'), the Midrash, interpreting blackness as relating to sin and beauty as relating to Mitzvos, asks how one can be black (with sin) but, at the same time, beautiful with Mitzvos. The Midrash answers with a parable: A king once punished his daughter for misbehaving and sent her to work in the cornfields. However, when the king found out that his daughter was, in truth, innocent, he made it up to her by reinstating her to an even greater position in the kingdom. This, says the Midrash, is like what happened to Bnai Yisrael. They sinned repeatedly, yet after each time, they did Teshuvah and became even greater for the whole experience. The Midrash thus says that Bnai Yisrael sinned and were "blackened" with the gold that they gave for the Golden Calf, but now they became beautiful by giving gold for the Mishkan. According to this Midrash, the "willing heart" of our Posuk was meant not to denote a limited number of people, but rather, it indicated the generosity and willingness of Bnai Yisrael as they wanted to contribute to the holy task of building the Mishkan.