In the beginning it's great to work in Japan, but at some point, you begin to think longer term. You may get the feeling you're just working in a job that really doesn't use your skills and won't boost your career as much as a job back home could, simply for the novelty of being able to live in Japan. Everyone tries to find the right balance for themselves.
Dude, have you been reading my mind???
Sometimes people leave Japan at this point. Sometimes a new company or a new type of work in Japan can re-invigorate and give you better job satisfaction. Other times, you'll decide a less stimulating job is offset by all the fun you're having in Japan.
This is probably the biggest issue you will face working in Japan (eventually). Look at it this way, once you get over all the languange and cultural hurdles to adjust to working and living in Japan, and your Japanese becomes perfect, you realize you have just become Joe Schmoe salaryman.
Holy shit, that's what has happened to me. I realized I became Joe Schmoe salaryman when two things happened: 1) when several Japanese co-workers and I were hanging out in the smoking room (as many companies here have, so that their nicotine habit doesn't destroy the rest of the office), and they all conceded to me at once that they were salarymen, and that since they were my co-workers, I must be a salaryman too, and 2) when I was riding the subway home from work wearing my suit and raincoat and carrying my briefcase and umbrella, and a Japanese friend of mine coincidentally ran into me on the train--she said she almost couldn't discern me from the other salarymen on the train, as if I blended in perfectly.
Oh shit, that's a FUCKED GAIJIN MOMENT! Ahhhhhhhhh.......
