Here's a picture of my classroom. I know what you're thinking, it's not quite as nice as the classroom you had at your university. Perhaps you're saying that because of the retractable walls (nearly the same one my high school gymnasium had), or because of the 1970s rug, or because of the random pieces of paper I've taped to the wall. Yeah, I'll concede that it isn't exactly an American standard lecture hall. That said, it totally serves its purpose.
At my university there are 11 periods every day. Each one of my classes is 40 minutes long. On one hand this is great because after being used to teaching 1.5 hour long classes, 40 minutes flies by. On the other hand, I teach up to 8 classes every day which is almost double the most I ever had to teach in Vietnam. It can be pretty exhausting. The other tough thing is that the students don't come to the same period every day. The students can basically come to whichever time slot they have free on that particular day. This means I've had classes with 12 students as well as classes with just one student. Needless to say, this presents some interesting challenges.
There's actually a lot of challenges that I've noticed in my first week or so. For one, my classes are 99.9% male only. I'm teaching at the engineering school which is probably 90% male and literally none of my classes have guys in them. On occasion I teach a high level class and those have a couple of girls. Having a room of all dudes is very different than my mixed classes I had in Vietnam. Also engineering students generally don't tend to be Type A personalities. It would be fair to call the majority of them introverted (actually, not having girls in the class I think helps in this regards because they aren't as embarrassed to talk). Getting students to talk and have fun is a little more of an effort than it was in Vietnam.
Another challenge has been the extremely different pronunciation problems that the Japanese have versus the ones that the Vietnamese have. For example, the Japanese have a very tough time saying the phonetic /v/ and /l/ sounds. My ear had become very well trained in Vietnam to understand Vietnamese broken English. Here in Japan, I constantly finding myself asking students to repeat things because I had no idea what they said. It's gonna be a challenge, but after week I can already hear a big difference in a few of my students. I think after three months there will be some very big strides...despite the gymnasium classroom.