Rabbi Hershel Schachter
About two years ago I came across a "teshuva" written by a Conservative clergyman. The thrust of the essay was that since the Tanoim established the halacha that women are exempt from wearing Teffilin because they are exempt from learning Torah, and today we expect women to learn Torah just like men, therefore women are no longer exempt from wearing Tefillin.
Needless to say, this is totally incorrect. The halacha that was formulated by the Tanoim that women are exempt from learning Torah has never changed. The laws of the Torah are not subject to change; the immutability of Torah is one of the thirteen principles of faith of the Rambam, and in our generation it has become the main point of distinction between Orthodox Judaism and other branches of Judaism. For centuries Orthodox women have been volunteering to shake a lulav on Succos and to listen to shofar on Rosh Hashonah. No one has changed the halacha that women are exempt from lulav and shofar, rather women have been volunteering to observe these mitzvos as an ainah m'tzuvah v'osah. In the days of the Bais Hamikdash only men were obligated to give machatzis hashekel towards the purchase of the korbonos tzibbur but the mishnah records that a woman may volunteer to observe the mitzvah as an ainah m'tzuvah v'osah.
We don't recommend in all cases that one volunteer to perform a mitzvah that he is exempt from. The Shulchan Aruch quotes from the Talmud Yerushalmi that if it is raining on Succos and sitting in the Succah would be very uncomfortable, not only is one exempt from the mitzvah, but also it simply does not make any sense to volunteer to observe the mitzvah - when sitting in the Succah is very uncomfortable there is simply no kiyum ha'mitzvah. If the lights in one's Succah have on gone out on the evening of Shabbos or Yom Tov and eating in the Succah would be very uncomfortable, but one's friend has a Succah a one hour walk away, one would not be obligated to walk for an hour in order to sit in the Succah. Nonetheless, if one did go out of one's way and walk for an hour, when one finally arrives at the friend's Succah and sits there comfortably, Rav Akiva Eiger says that one may recite the brocha of leishev baSuccah. In this instance, the one who walked the hour is volunteering to observe the mitzvah in a fashion of aino m'zuvah v'oseh.
Rabbi Soloveitchik, who gave a shiur on Gemorah in Stern College, did not intend to disagree with the Talmudic principle that women are exempt from talmud Torah. He merely felt that in that generation it made good sense that the opportunity should be available for women to volunteer to study gemorah, in the same way that women have been volunteering for centuries to observe lulav and shofar. At that time he recommended that the gemorahs studied by women should not be Maseches Baba Kamma or Maseches Sanhedrin, but rather Maseches Brochos, Perek Kol Ha'bosor, Maseches Shabbos, etc. which discuss dinim that are relevant to women halacha l'ma'aseh.
The Ta'noim understood from a phrase in the beginning of Parshas Vayikra that the mitzvah of semicha (i.e. that the one who brings a korbon must lean on the head of the korbon before sh'chitah) only applies to men and not to women. The expression "Bnai Yisroel" which appears in chumash so many times sometimes comes to exclude geirim (converts), sometimes comes to exclude women, and sometimes excludes neither. The Tanoim had a feel and a sense for how to darshon the pesukim based on the context of the passuk.
During the period of the second Bais Hamikdash, many women felt bad that they were not permitted even to volunteer to do this mitzvah of semicha since doing so would be a violation of avodah b'kodshim (getting work/benefit from a korban by the korban supporting their weight when they lean on it). Men who are obligated to do semicha are obviously not in violation of this prohibition of avodah b'kodshim, but since women are not obligated to do semicha, were a woman to do it voluntarily she would be in violation of this issur. As a result, many women wanted to perform an "imitation semicha" (i.e. without actually leaning on the head of the animal but merely by having their hands float on top of the head of the animal). The permissibility of this was a big dispute amongst the Chachomim. Many were of the opinion that the performance of such an "imitation semicha" might possibly lead mistakenly to a violation of avodah b'kodshim if women would actually lean on the animal, and therefore it should not be permitted. The accepted opinion is that we do permit it, but we have to be careful that one thing should not lead to another.
The bottom line is that each of us has to observe all mitzvos that we are obligated in. However, when it comes to someone volunteering to do that which is not obligatory on him/her, there are rules and regulations pertaining to each individual mitzvoh/halacha specifically, and to observance of halacha in general, and it is not so simple to determine when one should or should not go beyond that which is obligatory.