Why did he specifically mention lighting a fire over and above all the other Melachos?
Rashi #1 and Ramban #1 (citing Shabbos, 70a): According to R. Yossi, it is to teach us that lighting a fire is only a La'av and is not subject to Kares.
Rashi #2 and Ramban #2 (Ibid.): According to R. Nasan, it is to teach us that one is Chayav on Shabbos for each Melachah that one transgresses - and not just if one transgresses all thirty-nine.
Ramban and Rashbam: To teach us that the prohibition of Melachah on Shabbos extends even to Ochel Nefesh. 1
Seforno: To teach that even though lighting fire is normally a Kilkul, for which one is generally Patur on Shabbos, one is Chayav. The reason that one is Chayav in the case of lighting a fire is because lighting a fire is needed for the performance of all, or most, other Melachos.
Torah Temimah (citing Shabbos, 20a): It teaches us that the prohibition of lighting a fire (and burning on it) is confined to private use, but is permitted with regard to burning the fat-pieces and limbs of Korbanos.