Haha, I'll just post the damn thing. Seriously, I'm not writing anything mindshatteringly amazing or provocative. I'd actually rewrite a lot of this, but whatever here it is unedited:
(Everything below is simply my personal opinion from personal experiences. I apologize for the length. Background: Lived in 6 different prefectures from 02-03, and 05-12, wife was a manager of GEOS when we met)
I've probably made a bunch of random posts on the topic over the years, but never really a single, concise post. Just to be clear, I didn't know much about the English teaching industry before I went to J-land to work. The head of my Asian studies department at school happened to be a member of the JET committee for the NYC consulate, and some of my classmates did end up doing it, but it was never really a thought for me.
After living there for so many years, I have had many experiences with the foreign teacher population. In general, my opinion is the majority of people who do English teaching take advantage of the entire situation for their own personal gain. They are usually not there to pursue knowledge or experience in the educational realm, nor there to promote their native culture and genuinely exchange information with the locals. They are usually there for an entirely different reason, which will be accomplished in a short amount of time, and then they leave. Those that do stay (whom we usually call lifers) simply realized that they had such a good situation that it was ridiculous to think of going back to their home country where they were almost surely going to be making less money while doing more work. Some also really get used to the "rock star" lifestyle they have there, and some find that living in Japan "fits" more with them than their home country (usually introverts).
Here are some reasons I've seen people use as an excuse to come "teach" in Japan: pay off loans, stop-gap between real job, stop-gap between grad school, want to party, want to travel Asia, no idea what to do, want to play games and watch anime, cannot get women in their home country, cannot find job in their home country, want to become fluent in Japanese.
Just to be clear, it is VERY easy to live and teach in Japan. You need a college degree. The end. You do not need any teaching background or experience whatsoever. How could anyone possibly be a teacher like that? Easy, you know English, so just teach it like you speak it.
How do you get there? You apply to a job, whether JET, an eikaiwa, an international school, whatever. If you get the job, they sponsor you, you come, that's it. The issue with "teaching" in Japan is there's no uniformity. Some people will actually teach, some people will never remotely do anything near teaching. They'll still try to convince you that they "work" because they prepare lessons! and make worksheets! and help kids after class! but they do that shit because they have to. The state of English teaching in Japan is honestly abysmal and completely dependent on how that specific school in that specific location handles it. Not to mention SO MANY of the English teachers there never take the time to learn Japanese. They waste their time doing so many other "extracurricular" things that they can't handle the simplest of life's tasks and burden others. It should be obvious I have been asked countless times by English teachers to help them do something because they had no idea how to.
Some people will say JET is more of a cultural exchange, which can definitely be true. I met a JET from Canada who was a music teacher at home. The stories I heard about this guy were amazing. He left his job and came to study Japan's education system. He put his heart and soul into his lessons. Then I met the majority of his "co-workers" from the same prefecture who did nothing but bar crawl the same 3 pubs every weekend, or set up rotating house parties at their places, or used every single last second of their nenkyuu to hit up a new a southeast Asian country every month. And then they left. Teaching in Japan was just their way of funding all that.
Honestly I could go on, and on, and on through my years there with stories that range from sad to insane, but this post is long enough. To sum it up, I think most foreigners teaching in Japan are taking advantage of the system and use it as either a stepping stone to accomplish their goals (whether that's to transition to a different field or simply live in Japan forever) or that use it as a way to chill and hang out while they save some money. If there was any other qualification than "native English speaker" than I'm sure my opinion would be much different. Alas, there are plenty of prominent posters on this forum who have taught English in Japan. I'm sure they are wonderful people with good intentions. But I wonder what their underlying motive was to go in the first place. Or, if they're still there, why are they still doing it? Married a local and can't do anything else due to language is the usual response. Hence, English teaching is the default way of supporting themselves, rather than something they actually want to do.
Japan really needs to reform the entire system. And I do apologize to the many, many teachers that are actually doing a wonderful job there. This post is obviously filled with many generalizations. But, in my experience, the amount of people taking that job seriously are much fewer than those that simply do it because they have to. If you want to go teach eventually, just ask yourself why you're doing it, especially if education is not even the field you plan to be in.
TLDR: Be a CIR instead!