The only significant practical difference between communal customs regarding the laws of the Seder is the question of whether to recite “ha-gefen” over the second and fourth cups.
Many Rishonim maintain that “ha-gefen” must be recited over each of the four cups, even though our attention is not diverted from one cup to the next, because each cup is a mitzva in its own right. This is the opinion of R. Natronai Gaon, R. Amram Gaon, Rif, Rambam, Maharitz, and Rema, and it is the practice of Ashkenazim and of Yemeni Jews who follow Rambam.
However, Rosh maintains that “ha-gefen” must only be recited before the first and third cups. The berakha over the first cup covers the second cup because there is nothing between them to divert our attention. We recite a berakha over the third cup because it follows Birkat Ha-mazon and a berakha is always recited over wine we drink after Birkat Ha-mazon, even if “ha-gefen” was recited earlier in the meal, because Birkat Ha-mazon serves as a berakha aḥarona for the wine one drinks during the meal. The berakha over the third cup covers the fourth cup as well. R. Yona and Rashba also maintain that “ha-gefen” is recited over the first and third cups only. SA rules accordingly, and this is the Sephardic custom.
There are also differing opinions among Rishonim regarding the berakha aḥarona over the wine. In practice, however, there is a consensus not to recite a berakha aḥarona after each cup of wine. Rather, Birkat Ha-mazon covers the first two cups, and the berakha aḥarona (“al ha-gefen”) recited after the fourth cup covers both the third and fourth cups.
Indeed, because of all of these problems, some poskim maintain that one should be careful to recite the Hagada in less than 72 minutes (see Sidur Pesaḥ Ke-hilkhato 3:9). Yet there are righteous and pious people who actually enhance the experience by prolonging the recitation of the Hagada beyond 72 minutes, explaining this practice with various rationales, such as explaining that the berakha aḥarona on the fourth cup also covers the first cup. (See Mikra’ei Kodesh 2:30, Hilkhot Ḥag Be-ḥag 20:5, and Responsa Ḥazon Ovadia §11.) In my humble opinion, the entire issue of a significant interruption (“hefsek”) in a meal only applies when one engages in unrelated activities (even if he does not take his mind off the meal). In this case, though, people are involved in the Seder and the Hagada, and the second cup is poured right at the beginning of the recitation of the Hagada, and since the entire recitation of the Hagada is connected to the second cup, the time between the first and second cups is not considered a hefsek. Consequently, there is no hefsek between kiddush and the meal; the recitation of the Hagada is considered part of the Seder, since telling the story of the Exodus is an integral part of the mitzva of eating matza. Similarly, telling the Exodus story is also inherently connected to the mitzva of eating maror, so the recitation of the Hagada does not constitute a hefsek between the eating of karpas and the eating of maror (and the berakha on karpas covers the maror as well).