1. It must be ten minutes long. Less and you are a major disappointment. More, and you are treading on everyone's patience. This is the minyan people go to because they want to daven without nigunim and get finished in time to make kiddush before 9:30 (it starts 7:15). One may slop a bit over, but you lose serious style points for each additional minute, as marked by the growing number of glares.
2. It must be on halacha (law), relating to the parsha (or some other attribute of the specific shabbas). It may not be pure homiletic or moral instruction. However, extra style points are awarded for having significant halachic content and significant homiletic content to make it more than just pure halacha.
3. There are a lot of people there with the first name "Rabbi."
In other words, there is absolutely no faking it. It requires serious research, and delivery in a compressed form. To make matters worse, I was asked Simchat Torah if I would do the drash on Shabbos Noach. I said "sure," mentally thinking I had two weeks (since Breishit comes first, a fact a surprising number of people got wrong on the PEW religious knowledge survey. Only after saying yes did I realize I only had one week (as Bereishit came the next day).
Still, I pulled something together on eiver min hachai (the command not to rip the limb from a live animal). I am given to understand it Did Not Suck. We shall see if I get asked again.
Most people assume that 7 commandments were given this week. In fact, as Rashi explains, only one of the Sheva Mitzvot B'nei Noach (7 Noachide Laws) was given this week -- eiver min hachai. The other six were, in fact, given to Adam and Chava. Indeed, as Rashi explains, Adam and Chava were initially prohibited from eating meat [which raises a subsidiary question of why Hevel and Noach brought animal sacrifice, but I did not address this and have no idea how to answer]. However, God explicitly permits meat (all manner of animal flesh) to Noach and his descendants in this week's parsha, necessitating the giving of eiver min hachai. This derives from Gen 9:4 "But be strong, not to eat the blood, for the blood is life."
Curiously, non-Jews are not, in fact, prohibited from eating blood. They can eat blood sausage and blood pudding to their heart's content. But not traditional Kumis, which is mare's milk fermented with blood cut from the living mare. As Rashi further explains, the law applies only when the limb has "nefesh," life.
The law is simple and straightforward in its application. A non-Jew is prohibited from eating a limb or section torn from a living animal of any kind. Nor is there a minimum shiur (measure) that must be consumed. The tiniest scrap of bruno (an Ethiopian dish where beef is cut out of the hide of a live cow) is enough to be liable. The law applies to all creatures, whether they technically have "blood" so long as they have life. So this includes fish and insects, but not "creeping things" (shratzim) that may fall into this category. The punishment for violation is death.
Curiously, this is not the source of eiver min hachai for Jews. As the Gemarah in Mesechet Chullin Perek Gid Hanasheh explains (Daf 101b-103b), for Jews, the source of eiver min hachai is found in Deut 12:23. "Only be steadfast in not eating the blood, for the blood is the life, and you shall not eat life with flesh." This separate derivation leads to several difference between eiver min hachai for Jews and non-Jews. For Jews, eiver min hachai is a negative commandment, punishable by lashes rather than death. It applies only to kosher animals for which schechitah (ritual slaughter) is required. Thus, fish and insects may be torn apart alive and not violate eiver min hachai. However, such conduct violates other negative commandments. Indeed, it is worth noting that nearly all food prohibited to non-Jews via eiver min hachai is prohibited to Jews via other commandments. Further, while the command does not prohibit the actual rending of a live animal, a Jews is prohibited from giving a non-Jew eiver min hachai to eat under the basic principle of "do not place a stumbling block before the blind." [Source: Mesechet Pesachim 22b]
This leaves us with a difficult question. It would appear that the Brit (covenant) of Sinai displaces the Covenant with Noah. Why should this be? Surely the Covenant of Sinai should be cumulative! But even if we assume that the Covenant of Sinai replaces the Covenant of Noah, how can it be that the Covenant of Sinai is less severe? Surely we would expect that as the Chosen People who accept the Yoke of Heaven in the form of the law, that our burden to fulfill the commandments would be greater than the burden on the other Nations of the World.
The answer lies in the purpose of the Law. God gives the law not as a burden. Rather, God intends that through his law, we should perceive the nature of His Will and thus the metaphysical nature of the universe. Our study and practice of the law in its entirety allow us to live with nuance and gradation necessary for greater understanding and perception. For non-Jews, it is sufficient for them to have a few basic principles. This is not to denigrate the capabilities of non-Jewish souls, all of which have the potential to achieve the World to Come. However, for those whose souls (whether by birth or conversion) are trained to the practice of the Law, such blunt principles and stark penalties are insufficient to the purpose. No one who is observant of the law will lust to consume eiver min ha chai because it is "only" punished with lashes. Nor will they decide to chow down on the severed limb of a pig because it is "merely" a prohibited animal and not ever min hachai. However, in the greater understanding of the LAw that comes through the exploration and practice of the law, we gain a greater under understanding of God's intent for us in this world and His design for this World and the Next.