While it goes without saying that one should allocate time for Torah study on Shabbat, it is interesting to note that a number of halachic authorities are of the opinion that one should not engage in intricate or in-depth Torah study on Shabbat. This is especially true with regards to Talmud study which is known to be one of the more difficult areas of Torah study. Indeed, some authorities have argued that one who delves deeply into Torah on Shabbat is considered to have desecrated Shabbat through excessive exertion, mental agony, and even transgressing the prohibition of "borer" when reflecting on different theories and arguments. One may also want to consider avoiding learning completely new material on Shabbat.
One explanation for this unusual and intriguing prohibition is that one who studies Torah in-depth on Shabbat burdens himself in a way that detracts from the requirement to spend Shabbat engaged in pleasurable activities (oneg shabbat). We are told that Rav Zeira would interrupt those who spent too much time studying on Shabbat, insisting that they engage in more pleasurable pursuits. Furthermore, it is suggested that one who generally studies in-depth during the week may be in violation of the prohibition of engaging in weekday activities (uvdin d’chol) by doing so on Shabbat! According to this approach, one should preferably study Midrash, Aggada, or Mussar on Shabbat. According to the Rebbes of Chabad one should spend "two-thirds" of one’s Shabbat study time focused on chassidut. The Chafetz Chaim was said to have studied Chumash with commentaries on Shabbat.
Lawrence Kaplan posts: Rationalists such as myself and others were upset because Rabbi Enkin used the poetic and profound notion of a fetus studying Torah in the womb in such a crude and literal way. Here the idea that the nefesh yeterah will be asked what new Torah insights it learned on Shabbat is itself a poetic idea, which should be understood metaphorically, that is, to offer one possibility, that the special spirit with which one is imbued on account of the holiness of Shabbat endows one with an increased capacity of Torah study and hiddush, and one has not made proper use of kedushat shabbat if one fails to engage in such study and hiddush –or something to that effect.
Y. Aharon posts: I consider some of the issurim of chazal, based ostensibly on things like snakes and demons, to be essentially hygenic in nature. Remember that hygenic conditions were an uncommon occurence in olden days. Hence leaving food outdoors, under beds, or in outhouses was considered objectionable or forbidden. The concern was rational even if the ostensible reason given was fanciful. The halacha is, therefore, not really dependent on the reasons given.