I appreciate it, lets see if I can answer all of your questions.
How was the admission process?
The admissions process ... there's a lot of essays to write for the application, but they are very similar to the NSF GRFP application essays so I was used to it. Also, I would suggest you look at possible advisors as soon as possible and try to start communication with them to help make a decision. The perspective adviser will hopefully write you a letter of recommendation (required for the application) which I think is pretty amazing. Also, I did a lot of research on alternative scholarships, and sadly ... Americans can't apply for any of them until they have already been attending the school for about a year, but doing such research on your own may convince your perspective adviser that you are willing to go all the way. Students from other Asian countries have a better chance at getting alternative scholarships because there are a ton. Also, the admissions office is extremely professional and will reply to your emails withing 24 hours unless it is the weekend in Japan or a national holiday (incredible!).
Any minimum requirements?
I don't know about minimum requirements other than graduating with a BS or BA before you would start at the University of Tokyo. It probably looks good to have past research experience, but other experience is probably a plus. Perhaps teaching experience? This is something that you could email the admissions office about. I could be wrong, but asking them insightful questions will get your name to them. Graduate schools like to offer admission to people that will NOT decline an acceptance. Rejection hurts graduate schools.
Is there an oral/written examination?
Incredibly, your accepted application is considered equivalant to taking their qualification examination. So you get away without having to take it. If you can prove your abilities and desire to do research with them, you do not need to take the exam. Technically, I don't think any Japanese is required either; though, I bet it helps.
Are all courses in English?
Excellent question. I have spoken with my adviser several times and have concluded that sadly, this is not yet an attainable goal. For the first semester or two, this can be done, but there just aren't enough courses (in my department at least) for me to get away with this. That means I will eventually take classes in Japanese. However, I was told that some professors are nice enough to allow you to do homework and perhaps tests in English. This is probably a good thing though, because you will familiarize yourself with the Jargon of your field in Japanese, which will give you an edge after grad school.
I hope this helped. If you have any other questions for me, feel free to ask. Don't forget to read their "Prospectus" page because it details the entire application process. Also, don't be afraid to ask the admissions office questions, they are very nice people.
Cheers,
デレック