“And he (Moshe) saw the [golden] calf and [the] dances, and Moshe’s anger raged and he threw the tablets from his hands and he broke them at the bottom of (lit. under) the mountain” (Sh’mos 32:19). There is much discussion about why Moshe broke the Luchos, the tablets into which G-d carved the words He spoke publicly at Mt. Sinai, the “Ten Commandments.” One of the more famous explanations (see Sh’mos Rabbah 43:1 and 46:1, Midrash Tanchuma Ki Sisa 30 and Eikev 11, Avos d’Rav Noson 2:3 and Ibn Ezra) compares Moshe breaking the Luchos to an emissary of the king (or of one of his officers) who tears up a royal marriage contract after discovering something unsavory about the person he was supposed to deliver it to, as it would be much worse when the king finds out what happened if the marriage was official than if it wasn’t.
Several Roshei Yeshiva (see Rinas Yitzchok III on 32:27 and Iyun HaParsha #63, http://tinyurl.com/be3c9fr) ask how Moshe breaking the Luchos made a difference, since the prohibition against idol worship (including the consequences of violating the prohibition) had already been issued and was still in affect (as evidenced by the death penalty being carried out on the violators, see 32:27-28 and 32:35).
There are several notable differences between how things were before and how they were after the sin of the golden calf. When (chronologically) the Mishkan was commanded is the subject of much discussion, with some commentators suggesting that it only became necessary after the nation had sinned (see S’fornu on 25:9 and 20:21), or that some aspects were different because of it (see http://RabbiDMK.posterous.com/Parashas-Ki-Sisa-5772). Rabbi Yaakov (Lorberbaum) of Lisa (the author of the Nesivos HaMishput), in Nachalas Yaakov (Parashas Vu’eschanan), attributes the differences between the first set of Luchos (in Parashas Yisro) and the second set (in Parashas Vu’eschanan) to the decline in the spiritual level of the nation due to the sin of the golden calf. The Talmud (Bava Kama 54b-55a) says the word “good” was not included in the first set of Luchos so that the good that was intended should not be lost after they were broken. Whatever this means, it is obvious that breaking the Luchos was more than just a symbolic gesture, and permanently affected the covenant between G-d and the Children of Israel; otherwise, having “good” included in the first Luchos wouldn’t prevent them from also being included in the second ones.
There are several Midrashim that describe a virtual tug-of-war over the Luchos. Some (e.g. Tanchuma Eikev 11, Yerushalmi Taanis 4:5) describe G-d holding on (as it were) to one side of the Luchos–trying to prevent Moshe from taking them down from Sinai–while Moshe held on to the other, while others (e.g. Avos d’Rav Noson 2:3 and Sh’mos Rabbah 46:3) describe the 70 elders trying to grab the Luchos away from Moshe to try to prevent him from breaking them. These Midrashim cannot mean that there was an actual tug-of-war over the Luchos, as G-d doesn’t have physical hands to grab onto one end, and if He really wanted to take the Luchos back from Moshe, He obviously could have. Also, there doesn’t seem to be any communication or contact between Moshe and anyone other than Yehoshua until after he broke the Luchos (see 32:19); even though he did it “before their eyes” (D’varim 9:17), making sure they saw that he broke them, he did it “under the mountain,” and only Yehoshua was with him until they reached the camp. Rather, the Midrashim are trying to convey the idea that G-d didn’t really want to let Moshe take the Luchos with him (either to prevent him from giving it to the nation or to prevent him from breaking them) and the nation (or at least its leaders) didn’t want Moshe to break them and destroy the symbol of their covenant with G-d.
Although the leaders may not have realized that the Luchos (and what they represented) could be replaced, and therefore tried to stop Moshe from breaking them, G-d had to know that they could, or at least that the nation couldn’t live up to the ideals that the first Luchos represented (which is why He wanted to wipe them out and start anew, see Sh’mos 32:10 and D’varim 9:14). Why did G-d want to, at least initially, keep the Luchos in heaven rather than just instructing Moshe to break them? I would suggest that the message G-d was trying to send by not letting Moshe take the Luchos right away was that He still wanted the ideal situation that the Luchos represented to exist, at least conceptually. By taking the Luchos and breaking them, Moshe was sending the message that this ideal is too much to expect from mere mortals (bear in mind that after accepting the Torah until the sin of the golden calf, death would not have applied to the Children of Israel, see Sh’mos Rabbah 51:8). The breaking of the Luchos signified a change from the ideal relationship that couldn’t be sustained to the possibility of a more realistic relationship, one that Moshe was able to get the nation ready for after helping them recover from their sin, was able to convince G-d to accept, and was represented by the second set of Luchos.
It is interesting to note that although after descending Mt. Sinai Moshe “drew nearer to the camp” (32:19), he broke the Luchos “at the bottom of the mountain.” Our sages connect this “bottom of the mountain” to the place where Moshe had built an altar and 12 pillars when the covenant was enacted (24:4, see Bamidbar Rabbah 9:48), although it is unclear whether Moshe broke them at that spot to signify that the covenant itself was still intact or that it was no longer intact. (Some suggest that he did so to indicate that the sin was shared by all 12 Tribes, not just those who actually worshipped the golden calf.) A connection can also be made to the “bottom of the mountain” (Sh’mos 19:17, see also D’varim 4:11) that the Talmud (Shabbos 88a) says was literally “under the mountain,” as G-d “held Mt Sinai above them,” threatening them to either accept the Torah or be killed. By breaking the Luchos “under the mountain,” Moshe was telling G-d that He shouldn’t hold the nation completely responsible for not living up to a covenant that they really weren’t ready for, yet was forced upon them; the covenant was still intact, but the full consequences of not living up to its ideal form shouldn’t be insisted upon.
The king’s messenger tearing up the marriage contract didn’t preclude the royal wedding from ever taking place; the contract just had to be rewritten. If/when it was, it would be with the knowledge and understanding of what had occurred. By breaking the Luchos, Moshe ended any possibility of the nation (or anyone else) being held to the previous standard, one that included the entire nation deserving death for allowing the few to worship the golden calf, and then started the process that led to a more realistic expectation of what the covenant entailed. Even then, some still deserved the death penalty, but not the entire nation. Once this was accomplished, the next steps could be taken, allowing Moshe receive a second set of Luchos, which represented the covenant that is still in effect to this day.