Why is it that the 9th of Av is a full-day fast but the other post-Mosaic fasts are daytime-only fasts? What determined the length?
The Talmud rules (Rosh Hashana 18b) that the Temple related commemorative dates have 3 possible statuses: in a time of peace, they are holidays; in a time of oppression, they are fasts; in a time of neither peace nor oppression, they are optional fasts, with the exception of Tisha Bav which remains an obligatory fast. The Ramban (quoted in Beit Yosef OC 550) explains that nowadays we are in the last category, and the custom is to fast only in the daytime and not to refrain from the other 4 afflictions (washing, leather shoes, etc.). The custom can be this way because the fast is optional to begin with. Were we to revert to a time of oppression (who determines this is hard to say), then all the Temple related fasts would have the same rules as Tisha Bav.