The Torah does not provide simanim to distinguish between kosher and non-kosher birds. Rather, a list of non-kosher fowl is given, and one must act according to masorah (tradition) in determining whether the fowl he is eating is not among those on the non-kosher list. Regarding fish, no masorah is required at all, one need only check to see if that particular fish has fins and scales. With respect to eating animals, the Torah makes their kosher status dependent on two simanim: mafris parsa (having split hooves) and maalah gerah (the chewing of the cud). The Shach in his commentary to Yoreh Deah (80:1) mentions (in connection with eating a kosher beheima) that a masorah is needed. The simple meaning of the Shach is that to distinguish between a beheima and a chaya a masorah is necessary. Cheilev of a chaya may be eaten, as opposed to cheilev of a beheima, which is prohibited. Although the Talmud gives simanim to determine whether any given animal is a beheima or a chaya, the Shach recorded a chumrah, that a masorah is needed. The Chazon Ish (Y.D. end of siman 11) points out that the Chochmas Adam (36:1) understood the Shach to have said that not only is it necessary for a masorah to identify any animal as a chaya (as opposed to a beheima), but even to allow one to eat any given animal the minhag requires a masorah, over and above the Biblical need for being ma'aleh geira and mafris parsa.
In general, masorah plays a most important role in establishing the halacha. Ramabam writes that in his opinion, we ought to not simply establish every seventh year as a shemittah year, but rather must have fifty-year cycles, with the 7th, 14th, 21st, etc. years observed as shemittah, and the fiftieth year being blank. (The special mitzvos of yoveil, the fiftieth year, only apply when the majority of the world Jewish population is located in Eretz Yisrael). However, the Rambam (Hilchos Shemittah V'Yovel, 10:5) continues to say, that the Geonim who lived in Eretz Yisrael and observed the laws of shemittah, clearly followed the practice of simply observing every seventh year as shemittah, and did not leave the fiftieth year blank. Although Rambam thought that this does not make any sense, he said that this practice should nonetheless be followed because masorah is most crucial in determining what the halacha should be. We ought to assume that there certainly must be some good explanation for this practice, even though Rambam thought it did not make any sense at all. (After many generations and much thought, Rav Chaim Soloveitchik gave a most reasonable explanation for this view of the Geonim, which indeed has clearly been the practice for many centuries.)
In a teshuva written by the Ran he raised an interesting suggestion. The mishna tells us that a megillah written in languages other than Hebrew may be used for the purpose of fulfilling the obligation of mikrah megillah on Purim. The majority of the audience who listen to the megillah in every shul do not really understand the original Hebrew. Wouldn't it make better sense to have a kosher megillah written on parchment in the vernacular, so that everyone would be able to understand? To this the Ran points out that already in the days of the Talmud it was the case that many of the listeners did not understand the original Hebrew text of the megillah, and nevertheless the minhag for so many centuries has always been to read from a Hebrew text. This idea of writing a megillah in translation could have been implemented centuries ago, but never was. We ought to assume that there must be a good halachic reason why this was never done. (And indeed, the Shulchan Aruch quotes the idea that a megillah written in two different languages may not be kosher. Rambam had a reading in a toseftta in Gittin that a get written in two different languages is not kosher; there is no flow and no continuity from one language to the other. Similarly in the megillah, since we really do not know how to accurately translate, "Ho'achashteranim benei horamachim", our megillah would be partly in English and partly in Persian, and this would not flow.)
The Talmud (Menachos 32b) discusses exactly what type of a shoe must be used for the performance of the mitzvah of chalitza. In conclusion the Talmud states that our practice for centuries has been to allow "a sandal", so therefore, even if Eliyahu Hanavi should appear and tell us not to use a sandal, we would not listen to him on this matter. The centuries-old practice has established the halacha in an irreversible manner.
Years ago, a prominent rabbi in Eretz Yisrael came up with an original idea as to how to permit Kohanim to go to medical school (i.e., to come in contact with meisim (corpses)).Rav Moshe Feinstein published a teshuva pointing out that this rabbi's suggestion could have been implemented centuries ago, but never was. Therefore we must assume that there must be some good explanation as to why the suggestion is not correct. (Indeed, in my sefer - B'Ikvei HaTzon - I have published what I consider quite a reasonable rebuttal.) And even if Eliyahu Hanavi were to appear and express his opinion in favor of this rabbi's notion, Rav Moshe thinks we would not even follow him on this matter (Igros Moshe, Y.D. 3:155). A matter of halacha which has been accepted for centuries can not be overturned, unless one can demonstrate that there simply was an error involved from the very outset.
Now we know that the halacha was always very flexible. The Maseches Sofrim (16:5) writes that this is precisely why the Torah Shebaal Peh was not written down, so that nothing should be "etched in stone". The mishna often records the mishna achrona (the later tanaim) as reversing the pesakim of the mishna rishona (the earlier tanaim). Anyone familiar with the Mishna Berurah knows that the Chafetz Chaim has reversed many accepted pesakim of the Shulchan Aruch! "This is the way of the Torah." This itself is an integral part of the masorah, that there should always be room for chiddush!
The Talmud relates that for years it was assumed that all of the Beit Shean area was endowed with kedushas haaretz, and fruits grown there are tevel and must be tithed. After many years, Rabbi Yehudah HaNasi reversed this accepted practice, after ascertaining that certain parts of that area were never endowed with kedusha when Ezra and Nechemia returned to build the bayis sheni. The prevalent practice was simply based on an error. The Talmud adds as a footnote, that the moral of this story is that when a rabbi comes to reverse a time-honored practice, we ought not always reject it out of hand, but should always consider the possibility of a chiddush in halacha (Chullin 6b).
Nevertheless, we still assume that a centuries-old halachic position, accepted and observed universally by all of Klal Yisroel, does not lend itself to reversal. The tradition makes room for, and even encourages, chiddush, but not for shinui (see Nefesh Harav pg. 64). According to Rambam, the binding force of the Talmud is precisely due to the fact that it was universally accepted by all of Klal Yisroel.
The Tosefta (Megillah Chap. 3) records that theoretically, a woman should be permitted to get an aliyah (to the Torah), however the Rabbis did not allow this because of kvod hatzibbur. This has clearly been the universal practice in Klal Yisroel for close to two thousand years.
G-d has created all men b'tselem elokim (in the image of G-d) with all of the divine attributes innately contained within their souls, and has commanded us "v'holachto b'drachav" ("you shall walk in His ways") to preserve and maintain all of those divine middos (character traits). We are also told that we ought to serve as an ohr lagoyim (as a light unto the nations) (see Yeshaya 42:6.) "And when the other people of the world will see that we have succeeded in maintaining our tselem elokim, they will learn from us to be G-d fearing" (Devarim 28:10), i.e., they will realize that they also have it within their power to maintain that b'tselem elokim that they were endowed with.
Part of our obligation of v'holachto b'drachav, to imitate G-d, i.e. to preserve and maintain those divine attributes that were implanted within us, requires of us to lead private lives; not to be seeking the limelight; not to be loud in speech, in dress, or in action. Hakadosh Baruch Hu is described by the Navi Yehsaya as a "kel mistater". He hides from man (see Nefesh Harav pg. 281).
This concept is what is called tsnius; to lead a life of tsin'a - as opposed to a life of farhesia (public). Sometimes the Torah requires of us to compromise on our tsnius and to do things in a public fashion. We need a government; we need kohanim sacrificing korbanot in the Beis Hamikdosh; we must have tefilla b'tsibbur. Even when we are required to compromise on our middas hatsnius (privacy) and enter the public eye, the halacha tells us that som tasim alecha melech - melech v'lo malka, that women should always try to maintain their privacy. Let the men run the government. Let the men offer the korbanot in the Temple. Let the men serve as chazzan for the public prayer, and let the men read from the Torah in public. If we simply do not have any other choice, we would call upon women to run the government and read from the Torah. But if a woman were to run the government or read from the Torah, this would indicate that we had no choice in the matter, that from all of the men present we were unable to get enough of them to take care of these activities. This creates a problem of kavod hatzibbur.
The motivation to allow women to get aliyot is not because we don't have enough men to do the job. Some women are looking for empowerment. Receiving an aliyah which was traditionally viewed as an act of compromising on one's privacy, has been looked upon by the amei ha'aretz as an act of empowerment. Pushy individuals try to "grab the omud" and "grab maftir" whenever possible. This attitude is in outright violation of the entire principle of tsnius. Hakadosh Baruch Hu is a Kel Mistater, and always tries to be maalim Himself. Why should we even consider giving someone an aliyah for the sake of empowering that individual if this attitude is totally contradictory to our whole outlook on life?
Rebbe, who reversed the accepted position on Beit Shean, was known as an extremely humble individual. The Talmud (Sotah 49a) states that when Rebbe passed away, humility disappeared from the world! When such a humble individual comes up with an original chidush, we have to consider it seriously. Humility is always very crucial with respect to determining psak halacha. (The Talmud (Eruvin 13b) tells us that the halacha was generally accepted like Beis Hillel as opposed to Beis Shammai because Beis Hillel were more humble.) How much more so when one wants to be mechadeish to reverse an accepted position, we must be sure that the author of the original idea is not formulating his chidush shelo lishma - just to gain popularity or for some other ulterior motive. Although it is permissible, and even encouraged, for one to learn shelo lisham, for one to be mechadeish shelo lishma is not allowed (see pg. 26 in B'Ikvei Hatson).
Rav Moshe in his essay on the topic of the kohanim attending medical school writes that the fact that some "scholar," not particularly known for his strength in psak, published a paper in which he was prepared to permit a centuries-honored prohibition universally accepted by Klal Yisroel, would itself seem to indicate that the author of the paper probably belonged to that group of individuals who are gaas libam b'hora'ah (arrogantly enjoy deciding questions of Jewish law). To be mechadeish, one must have an extra degree of humility like Rebbe!
The Talmud records several disputes between the Tzedukim and the Rabbis. One of them was with respect to inheritance (Bava Basra 115b). The Tzedukim were apparently bothered with the fact that the Torah discriminated against women regarding the laws of yerusha(inheritance), and they attempted to "rectify" this "injustice" somewhat. In later years the early Christians adopted several of the positions of the earlier Tzedukim. The Talmud (Shabbos 116b. See Shaylos V'Teshuvos Tsafnas Paneach #313) records that the early Christians divided yerushos (inheritances) equally between sons and daughters. Several centuries later, the Reform movement continued with this complaint against the tradition, that the rabbis were discriminating unfairly against women by having them sit separately in the synagogue, etc. This complaint has developed historically to become the symbol of rebellion against our masorah. The fact that this symbolizes harisus hadas (destruction of the religion), causes it to become a prohibited activity (See Nefesh Harav pg.233 with respect to driving to synagogue on Shabbos). Rabbi Akiva lived at a time when many were attacking the Jewish religion. The Talmud records that rather than be apologetic about various Torah laws that didn't seem that reasonable in his generation, and rather than look for legal loopholes to get around those particular laws, (as some of our generation have suggested that "when there is a rabbinic will there is a halachic way"), he would rather take a different route. He would insist on the most stringent observance of b'dafka (specifically) those halachot which the "modern Jew" (of his day) felt most uncomfortable with, to prevent the religion from falling on the slippery slope (see Divrei Hashkafa p.72). Rav Soloveitchik zt"l wrote (Divrei Hashkafa p.233) that this indeed has been the traditional response of our rabbanim throughout the ages.