1) HALACHAH: AN ESROG "KUSHI"
OPINIONS: One Beraisa states that an "Esrog Kushi" is valid, while an Esrog which is "similar to a Kushi" is invalid. Another Beraisa, as well as the Mishnah (34b), state that an Esrog Kushi is invalid.
Abaye explains that when the Mishnah says that an Esrog Kushi is invalid, it refers to an Esrog "similar to a Kushi." An actual Esrog Kushi, though, is valid.
Rava argues and says that even an actual Esrog Kushi is invalid, as the Mishnah states, but it is invalid only when it is used in Eretz Yisrael. Since such an Esrog is not common in Eretz Yisrael, it does not qualify as "Hadar" and thus it is invalid. The Beraisa that says that an Esrog Kushi is valid refers to an Esrog Kushi when it is used in Bavel, where such Esrogim are common.
What is an "Esrog Kushi" and what is an Esrog which is "similar to a Kushi"?
(a) Rashi explains that an Esrog Kushi is one that grew in the land of Kush. The Kush-species of Esrog is much darker than the ordinary Esrog. Such an Esrog is valid for use in Kush because that is its normal habitat. In contrast, if a local Esrog tree in another place produces a mutant Esrog which is dark colored and looks like an Esrog from Kush (it is "similar to a Kushi"), it is invalid even in Kush. Rava adds that even the Esrog Kushi itself is invalid in Eretz Yisrael (if it was imported, for example), because such dark Esrogim are uncommon and are considered uncomely to those who are not accustomed to such dark Esrogim.
Why, according to Rava, is an Esrog Kushi valid in Bavel? Rashi explains that Bavel is close to Kush, and the Esrogim of Kush are thus commonly found in Bavel. (Support for Rashi's explanation may be found in the Yerushalmi (Sukah 3:6).)
The land of Kush is commonly identified as Ethiopia. If Kush is Ethiopia, though, why does Rashi say that it is close to Bavel? Ethiopia is closer to Eretz Yisrael than it is to Bavel, and thus its Esrogim should be more common in Eretz Yisrael than in Bavel.
Apparently, the land of Kush mentioned in the Mishnah and Beraisa does not refer to the Kush in east Africa. Rather, it refers to a country close to Hodu (India), as mentioned in the Gemara in Megilah (11a). (This Kush was likely in the region of the Hindu-Kush Mountains in east Afghanistan).
(b) RABEINU CHANANEL, the RIF and RAMBAM (Hilchos Lulav 8:8) write that an Esrog Kushi refers to a dark green Esrog. Such an Esrog is valid in Bavel, because in Bavel the Esrogim are darker than in Eretz Yisrael. "Similar to a Kushi" means that it is a much darker color, closer to black. The words "similar to a Kushi" do not mean that the Esrog is similar to an Esrog from Kush, but rather that it is similar to the skin color of a native person of east Africa ("Kushi"). Even in Bavel such an Esrog is invalid because such Esrogim are not common. In Eretz Yisrael both types of Esrogim are invalid, because the Esrogim there are generally of a lighter green.
HALACHAH: The Rif and Rambam are more stringent than Rashi with regard to an Esrog which is very black. According to their interpretation of the Gemara here, even where Esrogim naturally have black skin, they are invalid. (Apparently, they maintain that a black Esrog is considered a different species.) Rashi, in contrast, permits such an Esrog in places where it grows and is commonly used. The SHULCHAN ARUCH (OC 648:17) rules like the Rif, that a very black Esrog is invalid regardless of where it is used or where it grows. (Today there is no practical difference between the different opinions with regard to a black Esrog, because there is no place in the world in which an Esrog naturally grows black.)
Rashi is lenient in another respect as well. Even in a place where Esrogim do not grow dark, if dark Esrogim are often imported to that place one may use the foreign Esrogim. Since they are a common sight in that place, they do not lack the condition of "Hadar." In contrast, the Rif and Rambam do not permit such an Esrog; they require that the Esrog look like the other Esrogim that grow in that place. However, the PISKEI RID, who explains "Esrog Kushi" like the Rif, nevertheless accepts this leniency and rules that when one is near the place where the dark Esrogim grow, he may use a dark Esrog for the Mitzvah. The Shulchan Aruch does not comment about the use of a dark Esrog in a place where such Esrogim are imported. (See BI'UR HALACHAH, DH Makom, who interprets the Machlokes Rishonim somewhat differently. Therefore, there may be grounds to be lenient and permit such Esrogim.)
2) AGADAH: THE DEEPER SIGNIFICANCE OF AN "ESROG KUSHI"
The Gemara discusses the validity of an "Esrog Kushi." One Beraisa states that an "Esrog Kushi" is valid, while an Esrog which is "similar to a Kushi" is invalid. Another Beraisa, as well as the Mishnah (34b), state that an Esrog Kushi is invalid. (See previous Insight.)
According to Rava's answer, even an actual Esrog Kushi is invalid, as the Mishnah states, but it is invalid only when it is used in Eretz Yisrael. Since such an Esrog is not common in Eretz Yisrael, it does not qualify as "Hadar" and thus it is invalid. The Beraisa that says that an Esrog Kushi is valid refers to an Esrog Kushi when it is used in Bavel, where such Esrogim are common.
The CHASAM SOFER (CHIDUSHEI CHASAM SOFER on Sukah) presents a fascinating homiletical interpretation of the Gemara's discussion about the validity of an Esrog Kushi (according to Rashi's definition; see previous Insight):
The Torah refers to Tziporah, the wife of Moshe Rabeinu, as a "Kushis" (Shemos 12:1). The Gemara in Moed Katan (16b) explains that the verse does not mean that her skin color was black, but that she was outstanding in her deeds.
According to the Midrash, the Esrog represents the Tzadik. Just as the Esrog both tastes good and smells good, the Tzadik performs good deeds and learns Torah. Since the Gemara in Moed Katan defines "Kushi" as "outstanding in one's deeds," an Esrog Kushi refers to an exceptionally great Tzadik, one who involves himself exclusively in Torah and spiritual pursuits and avoids involvement with the mundane activities of the world.
The Gemara in Berachos (35b) records the famous dispute between Rebbi Yishmael and Rebbi Shimon bar Yochai. According to Rebbi Yishmael, one should learn Torah as his primary endeavor and work for a livelihood as his secondary endeavor. According to Rebbi Shimon bar Yochai, one should be involved exclusively in learning Torah. The Gemara says that many people conducted themselves like Rebbi Yishmael and met with success, while many others conducted themselves like Rebbi Shimon bar Yochai (they learned Torah to the exclusion of all other pursuits) but did not succeed.
The Chasam Sofer explains in the name of his Rebbi, the HAFLA'AH, that the reason why those who followed Rebbi Shimon bar Yochai did not succeed was because they followed his philosophy only superficially. They acted like him outwardly, while inwardly they lacked his conviction and his faith, and thus they did not succeed. They were not "Tocho k'Voro" (Berachos 28a) -- they were not as genuine on the inside as they appeared to be on the surface. They only "did like Raban Shimon bar Yochai."
The Hafla'ah explains that the Beraisa here which discusses the "Esrog Kushi" alludes to the conditions necessary to succeed in following the philosophy of Rebbi Shimon bar Yochai. A Talmid Chacham who is entirely "Kushi," externally and internally, is the only one who will succeed. If he is only "similar to a Kushi" -- he acts like a "Kushi" but inwardly he is not as committed as he appears, then he will not succeed.
The Chasam Sofer explains Rava's statement in the same way. Rava says that even an Esrog Kushi is valid only outside of Eretz Yisrael. Rava alludes to the opinion of Rebbi Yishmael in Berachos, who maintains that a person is supposed to be involved in worldly activities in order to earn a livelihood while he makes Torah learning his primary pursuit. The Gemara in Berachos relates that Rava himself would send away his Talmidim twice a year to earn a livelihood, because he followed the view of Rebbi Yishmael.
Rava here says that in Eretz Yisrael even a "Kushi" is invalid -- a Talmid Chacham should not be involved only in learning Torah, because he must work the fields and reap the produce of Eretz Yisrael. This requirement of a Talmid Chacham, however, applies only in Eretz Yisrael, because only in Eretz Yisrael does cultivating the fields involve a practical Mitzvas Aseh (Yishuv Eretz Yisrael). The Mitzvah of Talmud Torah does not override that Mitzvah, just as it does not override the Mitzvos of holding the Arba'as ha'Minim on Sukos and eating Matzah on Pesach. In contrast, outside of Eretz Yisrael, where there is no Mitzvah to cultivate the land, Talmud Torah overrides the need to be involved in earning a livelihood, and therefore in Bavel (outside of Eretz Yisrael) an Esrog Kushi (a Talmid Chacham involved only in learning Torah) is valid.