In his commentary on the opening possuk in Parshas Tzav, Rashi quotes from the Toras Kohanim that the term "mitzvah" has a technical connotation. It refers specifically to an obligation which is binding throughout all generations. From time to time in the past we had nevi'im who would instruct our people to perform specific horo'as sha'ah which were not intended to be of a lasting nature. These were never considered "mitzvos", technically speaking. Our tradition has it (Megilla 2b) that the only prophet who gave over "mitzvos", i.e. obligations which are binding throughout all generations, was Moshe Rabbeinu.
This does not mean to imply that every instruction of Moshe Rabbeinu was a "mitzvah". Many of his prophecies were also only intended as horo'as sha'ah. The instruction to knock on the next door neighbor's door on erev Pesach of the exodus and to ask for gifts was an obligation only once in the history of the world. The instructions regarding not leaving over the mohn only applied during those forty years while traveling in the midbor.
The Rambam (Yesodei HaTorah 9:2) understands the mishna in Sanhedrin (89a) as saying that one who violates the instructions of a novi deserves misa beyedei shomayim. The Minchas Chinuch (#516) is bothered with a most obvious problem: wasn't Moshe Rabbeinu a novi? It should therefore follow that anyone who violates any Biblical law given by Moshe Rabbeinu ought to deserve this punishment of misa beyedei shomayim! How can that be? The list of aveiros which warrant this punishment appears in the Talmud (Sanhedrin 83a), and is very brief. Why should Moshe Rabbeinu be inferior to other prophets, that "over al divrei navi" should apply only to the other prophets, and bedavka not to Moshe?
Various suggestions were offered over the years in response to this issue. (see shaylus u'teshuvos Tzafnas Paneach, by Rav Yosef Rozen, 138:5.) The generally accepted approach today is that which was offered by Rav Soloveitchik over sixty years ago (see Divrei Hagos V'Ha'aracha, pg. 66. See Minchas Yisroel, by Rav Yisroel Shurun, pg. 22). "Ho'over al divrei novi" only applies to one who violates a hora'as sha'ah. One who would leave over some mohn until the morning, in violation of the instructions of Moshe Rabbeinu, would indeed deserve misa beydei shomayim. But the "mitzvos", with a binding force for all generations, and which were only given by Moshe Rabbeinu, are not included in this category. They have their own system which has its own hierarchy of punishments. "Over al divrei novi" applies only to one who violates a "dvar nevuah". Some of Moshe Rabbeinu's instructions were "divrei nevuah", while most were elevated to the level of "divrei Torah" and "mitzvah" because of their binding force for all generations.
In the first half of the Sefer Hamitzvos, Rambam postulates what he considers the fourteen principles which he feels determine whether any given commandment deserves to be included in the list of the 613 mitzvos. His third guideline is that only obligations which apply throughout all the generations are considered mitzvos. This principle is rooted in the passage of the Toras Kohanim cited in the Rashi mentioned above.
In his commentary to the mishnayos (end of Sanhedrin), Rambam lists what he considers are the thirteen principles of our faith. We believe in prophecy. It is possible for G-d to communicate with man[1]. We also believe that the prophecy of Moshe Rabbeinu was on a higher level than that of any of the other prophets. What does this mean? Is Rambam grading the prophets? If Moshe Rabbeinu gets an A+, what does Micha get? And what grade does Chavakuk deserve?
No, this is not a matter of grading Moshe's prophecy. What Rambam means to say is that the only prophet who was ever given mitzvos (with a binding force for all future generations) was Moshe Rabbeinu. His was the only prophecy that was on the level of Torah.
This point is spelled out explicitly in Rambam's commentary to mishnayos Chulin at the end of Gid Hanoshe. Even the mitzvos of milah and gid hanoshe which were given to Avraham Avinu and to Yaakov Avinu are not binding today because of Avraham's prophecy, or that of Yaakov; but rather because these commandments were given again later on to Moshe Rabbeinu. Only then did they acquire the status of "mitzvos". Before ma'amad har Sinai, milah was only a "dvar nevuah", and one who would not fulfill this obligation would deserve misa beydei shomayim. This explains the incident recorded in Parshas Shmos, where the angel came to kill Moshe for neglecting to perform the milah of his son. At that time milah was not yet (strictly speaking) a mitzvah, and as a "dvar nevuah" one who would violate it would have the status of "over al divrei navi".
Another one of the Rambam's thirteen principles of faith is that the laws of the Torah are immutable. In recent years this has been a fundamental point of distinction between Orthodoxy and other groups.
This principle requires a bit of elaboration. Just because we believe in Torah min hashomayim, why does it necessarily follow that all the Torah laws are immutable? What would be so bad if G-d would notify us, by way of His prophets, that due to changing circumstances some of the mitzvos no longer apply? Why do we assume that any prophet who would deliver such a prophecy is automatically labeled as a navi sheker and deserves the death penalty (Rambam Yesodei HaTorah 9:4)? Why such an obstinate insistence on the part of the Orthodox that all the Torah laws are immutable?
The explanation is given by the author of the Tanya as well as Rav Chayim of Volozhin who both develop the identical theme. The Torah is not merely a collection of laws. In its entirety it constitutes a description of G-d's essence. Of course we can't really comprehend His essence. One of the Jewish philosophers of the middle ages commented that, "if I would understand Him, I would be Him." The only one who can understand Elokus is G-d Himself. Nonetheless, He gave us the Torah which by way of moshol (analogy) constitutes a description of Elokus. It is for this reason that the Torah is described as "The Moshol HaKadmoni", the moshol of Hakadosh Baruch Hu (see Rashi to Parshas Mishpatim 21:!3). Rav Chaim of Volozhin comments, it would probably be better to say that the Torah is a moshol of a moshol of Elokus, as opposed to assuming that it has a direct moshol.
The prophet Malachi (3:6) tells us that G-d's essence never changes. Everything in the creation is subject to change, but G-d the Creator never changes.
Since our tradition has it that the Torah is a description (even if only by way of moshol) of Elokus, and the prophet Malachi tells us that G-d's essence can not be affected by change, it therefore follows that the laws of the Torah can never change. The Torah (Beha'aloscha 12:8) distinguishes between the level of prophecy of Moshe Rabbeinu and that of the other prophets. Moshe was the only prophet "who was shown the image of G-d." What can this possibly be referring to? We believe that G-d has no body - there is no "image of G-d"!
What the possuk is driving at is exactly the principle we developed above. Moshe Rabbeinu was the only prophet who was given what we technically refer to as "mitzvos", commandments which are binding throughout all the future generations, because they constitute the description of G-d's essence, which is not subject to change. None of the prophets were ever shown "the image of G-d", i.e., were never given "mitzvos". They were only given a "hora'as sha'ah", of a temporary nature only.
The concept of "continuing revelation" developed by certain members of the Conservative movement is totally unacceptable. It is in clear contradiction to the Rambam's thirteen principles of faith which have been accepted. There certainly is a concept of "lifnim mishuras hadin", that one goes further than the halacha requires, all in the same direction as indicated by the Torah. But one cannot go contrary to the halacha and consider that lifnim mishuras hadin. The concept of "lifnim mishuras hadin" only applies when one is going in the same direction as the halacha requires, but even past the point of requirement. When one acts contrary to the din, this does not constitute chassidus.
There will always be instances where there will be a clash between two contradictory mitzvos. Life is always full of conflict! The world is always full of contradictions! Much of the halachic literature deals with how to resolve halachic conflicts. We must follow halacha even when it appears to us to be unethical or immoral. The Holy One who implanted within us the sense of ethics and morality is the same One who commanded us to follow His halachos, even if we don't understand them.
[1] Man's ability to communicate with G-d (by way of prayer) is also included in this principle: There is communication between G-d and man.