:Thanks, C S. I have a couple of comments. First, I would not describe the subject of the article as a "megacity" for a couple of reasons. One is that it pertains only to part of the subject of the article; it is quite incorrect for the mountainous western towns and the islands. The lead should clearly establish the subject of the article: Tokyo-to, not romantic notions of Tokyo the megalopolis. Another reason is that "prefecture" is an official, governmental designation whereas "megacity" is a conclusion drawn by others about Tokyo. The sentence mixes the formal and informal in a way that I find confusing.   
 The "megacity" is the Greater Tokyo Area; Tokyo is not the same as the megacity.  

 :Second, in my opinion, it would be an improvement to remove "are often informally considered" (which actually might have been my words; I've lost track) from the lead. How would you feel about this: "The twenty-three special wards of Tokyo, each governed as a city, cover the area that was once the city of Tokyo in the eastern part of the prefecture, and total over 8 million people. The population of the prefecture exceeds 12 million." 

 :Third, I'd like to leave the de facto/de jure discussion for later in the article. In the lead, the summary would be "Tokyo is the seat of the Japanese government and the Imperial Palace, and the home of the Japanese Imperial Family." 

 :That leaves us with something like the following: 
 , officially , is one of the 47 prefectures of Japan and located on the eastern side of the main island Honshū. The twenty-three special wards of Tokyo, each governed as a city, cover the area that was once the city of Tokyo in the eastern part of the prefecture, and total over 8 million people. The population of the prefecture exceeds 12 million. 

 Tokyo is the seat of the Japanese government and the Imperial Palace, and the home of the Japanese Imperial Family. The name "Tokyo" literally means eastern capital. 

 :Comments?